Specialize galera client config

For reliable operation, the mysql client configuration must point to the
local instance, within galera containers. However this prevents the
mysql client from working on other constainers (eg utility).

This patch defaults the mysql connection to the VIP, then overrides that
to 127.0.0.1 for galera containers.

Related: #413
Merge pull request #415 from hughsaunders/use_local_socket_in_client_my_conf

Specialise galera client config