new: allow relations to avoid creating any links.

This is done through a new ``tech-dep`` attribute in the
definition of the relation (done in the ``provide`` part of
the ``metadata.yml`` of a charm).

This replaces the ``reverse-tech-dep`` as it no allows 3 values.
The default one is ``True`` which simply allow the creation of
a ``link`` in the ``docker-compose`` file.
The value ``False`` will prevent creation of any ``link``, and the
``reversed`` value will reverse the ``link`` created.

bin/compose

@@ -212,12 +212,12 @@ _get_docker_compose_links() {
     master_charm=$(_get_top_master_charm_for_service "$service") || return 1
 
     deps=()
-    while read-0 relation_name target_service relation_config reverse; do
+    while read-0 relation_name target_service relation_config tech_dep; do
         master_target_charm="$(_get_top_master_charm_for_service "$target_service")"
         [ "$master_charm" == "$master_target_charm" ] && continue
-        if [ "$reverse" ]; then
+        if [ "$tech_dep" == "reversed" ]; then
             deps+=("$(echo -en "$master_target_charm:\n  links:\n    - $master_charm")")
-        else
+        elif [ "$tech_dep" == "True" ]; then
             deps+=("$(echo -en "$master_charm:\n  links:\n    - $master_target_charm")")
         fi
     done < <(get_compose_relations "$service") || return 1
@@ -580,7 +580,7 @@ get_charm_relation_def () {
 export -f get_charm_relation_def
 
 
-get_charm_reverse_tech_dep_relation() {
+get_charm_tech_dep_orientation_for_relation() {
     local charm="$1" relation_name="$2" cache_file="$state_tmpdir/$FUNCNAME.cache.$(echo "$*" | md5_compat)" \
           relation_def metadata value
     if [ -e "$cache_file" ]; then
@@ -589,14 +589,15 @@ get_charm_reverse_tech_dep_relation() {
         return 0
     fi
     relation_def=$(get_charm_relation_def "$charm" "$relation_name" 2>/dev/null)
-    value=$(echo "$relation_def" | shyaml get-value 'reverse-tech-dep' 2>/dev/null)
+    value=$(echo "$relation_def" | shyaml get-value 'tech-dep' 2>/dev/null)
+    value=${value:-True}
     echo "$value" | tee "$cache_file"
 }
-export -f get_charm_reverse_tech_dep_relation
+export -f get_charm_tech_dep_orientation_for_relation
 
 ##
 ## Use compose file to get deps, and relation definition in metadata.yml
-## for reverse-tech-dep attribute.
+## for tech-dep attribute.
 get_service_deps() {
     local service="$1" cache_file="$state_tmpdir/$FUNCNAME.cache.$(echo "$*" | md5_compat)"
     if [ -e "$cache_file" ]; then
@@ -607,7 +608,7 @@ get_service_deps() {
     (
         set -o pipefail
         get_compose_relations "$service" | \
-            while read-0 relation_name target_service _relation_config reverse; do
+            while read-0 relation_name target_service _relation_config tech_dep; do
                 echo "$target_service"
             done | tee "$cache_file"
     ) || return 1
@@ -923,19 +924,19 @@ get_compose_relations () {
                     case "$(echo "$relation_def" | shyaml get-type 2>/dev/null)" in
                     "str")
                         target_service="$(echo "$relation_def" | shyaml get-value 2>/dev/null)"
-                        reverse="$(get_charm_reverse_tech_dep_relation "$target_service" "$relation_name")"
-                        echo -en "$relation_name\0$target_service\0\0$reverse\0"
+                        tech_dep="$(get_charm_tech_dep_orientation_for_relation "$target_service" "$relation_name")"
+                        echo -en "$relation_name\0$target_service\0\0$tech_dep\0"
                         ;;
                     "sequence")
                         while read-0 target_service; do
-                            reverse="$(get_charm_reverse_tech_dep_relation "$target_service" "$relation_name")"
-                            echo -en "$relation_name\0$target_service\0\0$reverse\0"
+                            tech_dep="$(get_charm_tech_dep_orientation_for_relation "$target_service" "$relation_name")"
+                            echo -en "$relation_name\0$target_service\0\0$tech_dep\0"
                         done < <(echo "$relation_def" | shyaml get-values-0 2>/dev/null)
                         ;;
                     "struct")
                         while read-0 target_service relation_config; do
-                            reverse="$(get_charm_reverse_tech_dep_relation "$target_service" "$relation_name")"
-                            echo -en "$relation_name\0$target_service\0$relation_config\0$reverse\0"
+                            tech_dep="$(get_charm_tech_dep_orientation_for_relation "$target_service" "$relation_name")"
+                            echo -en "$relation_name\0$target_service\0$relation_config\0$tech_dep\0"
                         done < <(echo "$relation_def" | shyaml key-values-0 2>/dev/null)
                         ;;
                     esac
@@ -961,7 +962,7 @@ run_service_relations () {
         for subservice in $(get_service_deps "$service") "$service"; do
             [ "${loaded[$subservice]}" ] && continue
             # debug "  Relations of ${DARKYELLOW}$subservice${NORMAL}:"
-            while read-0 relation_name target_service relation_config reverse; do
+            while read-0 relation_name target_service relation_config tech_dep; do
                 export relation_config
                 Wrap -d "Building $DARKYELLOW$subservice$NORMAL --$DARKBLUE$relation_name$NORMAL--> $DARKYELLOW$target_service$NORMAL" <<EOF || return 1
 _run_service_relation "$relation_name" "$subservice" "$target_service" "\$relation_config"
@@ -1123,7 +1124,7 @@ has_service_action () {
         return 0
     fi
     ## Action provided by relation ?
-    while read-0 relation_name target_service relation_config reverse; do
+    while read-0 relation_name target_service relation_config tech_dep; do
         target_charm=$(get_service_charm "$target_service") || return 1
         target_script="$CHARM_STORE/$target_charm/actions/relations/$relation_name/$action"
         if [ -x "$target_script" ]; then
@@ -1237,7 +1238,7 @@ _get_master_charm_for_service() {
             ## Action provided by relation ?
 
             target_service=
-            while read-0 relation_name candidate_target_service _relation_config _reverse; do
+            while read-0 relation_name candidate_target_service _relation_config _tech_dep; do
                 [ "$interface" == "$relation_name" ] && {
                     target_service="$candidate_target_service"
                     break