Cleanup nova_url warning

Fix the logic to send a warning when nova_url is actually set.
Until now, the warning was showed all the time because we didn't check
is_service_default fact.

Change-Id: Iee721eb590d13e43c7b1458868eab441693a4425

manifests/server/notifications.pp

@@ -158,7 +158,7 @@ class neutron::server::notifications (
     fail('You must provide either tenant_name or tenant_id.')
   }
 
-  if $nova_url {
+  if ! is_service_default ($nova_url) {
     warning('nova_url is deprecated and will be removed after Newton cycle.')
   }