cluster_config argument missing in prepare()

Due to a bad merge, the prepare() call in postgresql does
not have cluster_config argument. Added that to make sure
postgres is in line with other data stores.

Change-Id: I1c3e5032447124dae47ae23ebf820fdcf8080588
Closes-Bug: #1380745

trove/guestagent/datastore/postgresql/manager.py

@@ -62,6 +62,7 @@ class Manager(
             config_contents=None,
             root_password=None,
             overrides=None,
+            cluster_config=None
     ):
         self.install(context, packages)
         PgSqlAppStatus.get().begin_restart()