fix: [mongo] use running mongo container with more accuray

Check the exact mount point whereas checking a parent dir allow
us to simplify the implementation of ``is_volume_used``. The new
implementation to be included in next version of compose is also
more solid (it doesn't rely on ``docker inspect`` that can stall
execution of the script if a container is stalled).

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

mongo/lib/common

@@ -9,7 +9,7 @@ export DATA_DIR=$SERVICE_DATASTORE$DB_DATADIR
 
 
 is_db_locked() {
-    is_volume_used "$DATASTORE/${SERVICE_NAME}"
+    is_volume_used "${DATA_DIR}"
 }