Add 'openstack-db' tag to db-sync Exec resource

In order to make easy orchestration on all OpenStack db-sync, add this
tag so people can use this tag in composition layer.
A use case it to set some orchestration to make sure MySQL Galera is
ready before running any Exec with this tag.

Change-Id: I1108bfeb4a82fb1b41e3eb135088940c052ce016
Closes-Bug: #1755102

manifests/db/sync.pp

@@ -31,6 +31,7 @@ class designate::db::sync(
       Anchor['designate::dbsync::begin']
     ],
     notify      => Anchor['designate::dbsync::end'],
+    tag         => 'openstack-db',
   }
 
 }