Fix mysqld backend check dependency

Waiting for haproxy mysqld backend should rely on
haproxy service for mysqld readiness.

Closes-bug: #1356748

Change-Id: Ib29c1cd981c4df2086050617320b56a70cacd50c
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>
Merge "Fix mysqld backend check dependency" into stable/5.0