Configuration for containerized MySQL clients

Configuration for containerized MySQL clients

This service generates the /etc/my.cnf.d/tripleo.cnf file which is
being used to configured MySQL clients (e.g. client bind address,
client SSL configuration...)

We generate the config file in this service and let containerized MySQL clients
mount /var/lib/config-data/mysql_client/etc/my.cnf.d/tripleo.cnf it in their
own container.  This way, when this MySQLClient service is updated, the other
containers will automatically pick the updated configuration at next restart.

Partial-Bug: #1692317
Change-Id: Idc56d27fb9645ad3b07df8ef08b7e2ce29e6d499