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

tasks/galera_server_backups.yml

@@ -88,7 +88,7 @@
     host: "%"
     priv: "*.*:RELOAD,PROCESS,LOCK TABLES,REPLICATION CLIENT"
     append_privs: yes
-    login_host: "{{ galera_address }}"
-    login_port: 3306
+    login_unix_socket: "{{ galera_unix_socket }}"
+    check_hostname: false
   no_log: true
   run_once: true