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: I901de48e1a8644499a7ab60c7d632cf17eee7e20
Closes-Bug: #1755102
(cherry picked from commit be770957ad)

manifests/db/sync.pp

@@ -27,6 +27,7 @@ class congress::db::sync(
       Anchor['congress::dbsync::begin']
     ],
     notify      => Anchor['congress::dbsync::end'],
+    tag         => 'openstack-db',
   }
 
 }

releasenotes/notes/openstack_db_tag-88dbf004beb21248.yaml

@@ -0,0 +1,3 @@
+---
+features:
+  - Add openstack-db tag to Exec that run db-sync.

spec/classes/congress_db_sync_spec.rb

@@ -17,6 +17,7 @@ describe 'congress::db::sync' do
                          'Anchor[congress::config::end]',
                          'Anchor[congress::dbsync::begin]'],
         :notify      => 'Anchor[congress::dbsync::end]',
+        :tag         => 'openstack-db',
       )
     end