Use unix socket while granting access for the backup service
Use unix socket while granting access for the backup service

/root/.my.cnf is not stored on galera container anymore, so it's not
possible to run mysql_user via TCP connection.
Unix socket should be used instead.

Change-Id: I71bc866aedaa6fba3cc86d2a2a99ed32f0727c54

openstack/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server