chg: database password file can now be stored in a different directory than host !api

``DB_PASSFILE`` is split into ``HOST_DB_PASSFILE`` and ``CLIENT_DB_PASSFILE`` to allow
to use a ``HOST_DB_PASSFILE`` in a directory that is accessible to ``compose`` even when
run in docker mode.

Using this, we can store HOST_DB_PASSFILE along with the database files,
which makes a lot more sense as it resolv issues of synchronising the
database with the content of this file (namely, the authorization
scheme). So when restoring the datastore of the database, the charm
scripts will be able to use the db password file and will not have to
worry about if it is in sync with the files or not.

Note that ``db_docker_opts`` and ``db_cmd_opts`` are now bash arrays. As
previous implementation was brittle, especially for handling spaces in
arguments.

bin/compose-core

@@ -560,10 +560,6 @@ ensure_db_docker_running () {
         fi
 
         docker_opts=
-        if [ -f "$DB_PASSFILE" ]; then
-            verb "Found and using '$DB_PASSFILE'."
-            docker_opts="$db_docker_opts -v $SERVER_ROOT_PREFIX$DB_PASSFILE:$DB_PASSFILE"
-        fi
         debug docker network create "$_DB_NAME"
         if ! network_id=$(docker network create "$_DB_NAME"); then
             err "'docker network create' failed !"
@@ -619,18 +615,20 @@ _dcmd() {
 
     debug "Db> $command $@"
 
-    if [ -f "$DB_PASSFILE" ]; then
-        verb "Found and using '$DB_PASSFILE'."
-        db_docker_opts="$db_docker_opts -v $SERVER_ROOT_PREFIX$DB_PASSFILE:$DB_PASSFILE"
+    if [ -f "$HOST_DB_PASSFILE" -a "$CLIENT_DB_PASSFILE" ]; then
+        verb "Found and using '$HOST_DB_PASSFILE' as '$CLIENT_DB_PASSFILE'."
+        docker_opts=("${db_docker_opts[@]}" "-v" "$HOST_DB_PASSFILE:$CLIENT_DB_PASSFILE")
+    else
+        docker_opts=("${db_docker_opts[@]}")
     fi
 
     ## XXXX was here: actualy, we need only connection between this version and the client version
     debug docker run -i --rm \
-           $db_docker_opts \
-           --entrypoint "$command" "$DOCKER_BASE_IMAGE" $db_cmd_opts "$@"
+           "${docker_opts[@]}" \
+           --entrypoint "$command" "$DOCKER_BASE_IMAGE" "${db_cmd_opts[@]}" "$@"
     docker run -i --rm \
-           $db_docker_opts \
-           --entrypoint "$command" "$DOCKER_BASE_IMAGE" $db_cmd_opts "$@"
+           "${docker_opts[@]}" \
+           --entrypoint "$command" "$DOCKER_BASE_IMAGE" "${db_cmd_opts[@]}" "$@"
 }
 export -f _dcmd