Added retries for db_sync
Added retries for db_sync

If db sync fails, it is never retried leading to errors. So, this
patch adds retries for db_sync to avoid sync fails.

Change-Id: Ia6a095c1bc0633a9369a7cf3f61d994d156ddd78
Closes-Bug: #1628580

openstack/puppet-gnocchi,openstack/puppet-gnocchi