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: I869ed64d19d82c64d6370a6b42c3016087f5760d
Closes-Bug: #1755102

manifests/db/sync.pp

@@ -30,5 +30,6 @@ class heat::db::sync(
       Anchor['heat::dbsync::begin']
     ],
     notify      => Anchor['heat::dbsync::end'],
+    tag         => 'openstack-db',
   }
 }

releasenotes/notes/openstack_db_tag-a3f0e6426366c25d.yaml

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

spec/classes/heat_db_sync_spec.rb

@@ -17,6 +17,7 @@ describe 'heat::db::sync' do
                          'Anchor[heat::config::end]',
                          'Anchor[heat::dbsync::begin]'],
         :notify      => 'Anchor[heat::dbsync::end]',
+        :tag         => 'openstack-db',
       )
     end
 
@@ -40,6 +41,7 @@ describe 'heat::db::sync' do
                            'Anchor[heat::config::end]',
                            'Anchor[heat::dbsync::begin]'],
           :notify      => 'Anchor[heat::dbsync::end]',
+          :tag         => 'openstack-db',
         )
         }
       end