MariaDB: create clustercheck user at container bootstrap

MariaDB: create clustercheck user at container bootstrap

In HA overclouds, the helper script clustercheck is called by HAProxy to poll
the state of the galera cluster. Make sure that a dedicated clustercheck user
is created at deployment, like it is currently done in Ocata.

The creation of the clustercheck user happens on all controller nodes, right
after the database creation. This way, it does not need to wait for the galera
cluster to be up and running.

Partial-Bug: #1707683
Change-Id: If8e0b3f9e4f317fde5328e71115aab87a5fa655f