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: I468f796bc344f91510e977dd07cfd563174c66dd
Closes-Bug: #1755102

manifests/db/sync.pp

@@ -19,6 +19,7 @@ class zaqar::db::sync {
       Anchor['zaqar::dbsync::begin']
     ],
     notify      => Anchor['zaqar::dbsync::end'],
+    tag         => 'openstack-db',
   }
 
 }

releasenotes/notes/openstack_db_tag-8af96261543c1429.yaml

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

spec/classes/zaqar_db_sync_spec.rb

@@ -19,6 +19,7 @@ describe 'zaqar::db::sync' do
                          'Anchor[zaqar::config::end]',
                          'Anchor[zaqar::dbsync::begin]'],
         :notify      => 'Anchor[zaqar::dbsync::end]',
+        :tag         => 'openstack-db',
       )
     end