allow to specify drivername for postgres db

in newer versions of sqlalchemy a drivername is required to
avoid a warning. For postgres the driver is psycopg2.
The driver is optional.

Change-Id: Iffc84d224fd9891261d787241687991fd97f48f7

manifests/db.pp

@@ -74,7 +74,7 @@ class keystone::db (
   $database_max_overflow_real = pick($::keystone::database_max_overflow, $database_max_overflow)
 
   validate_legacy(Oslo::Dbconn, 'validate_re', $database_connection_real,
-    ['^(sqlite|mysql(\+pymysql)?|postgresql):\/\/(\S+:\S+@\S+\/\S+)?'])
+    ['^(sqlite|mysql(\+pymysql)?|postgresql(\+psycopg2)?):\/\/(\S+:\S+@\S+\/\S+)?'])
 
   oslo::db { 'keystone_config':
     db_max_retries          => $database_db_max_retries,

releasenotes/notes/allow-drivername-for-postgresql-daa276a598844884.yaml

@@ -0,0 +1,3 @@
+---
+features:
+  - Allow to specify drivername for postgres db