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: Ife3a9bb2851fd730dbd693bda18733ffdec1286e
Closes-Bug: #1628580

manifests/db/sync.pp

@@ -24,6 +24,8 @@ class rally::db::sync(
     path        => '/usr/bin',
     user        => 'root',
     refreshonly => true,
+    try_sleep   => 5,
+    tries       => 10,
     logoutput   => on_failure,
   }