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

manifests/db/sync.pp

@@ -19,6 +19,8 @@ class gnocchi::db::sync (
     path        => '/usr/bin',
     refreshonly => true,
     user        => $user,
+    try_sleep   => 5,
+    tries       => 10,
     logoutput   => on_failure,
   }