Rename mysql to mariadb for registered var

To avoid confusing in code these changes rename a `register`
to `mariadb` rather than `mysql` since mysql service is not present
anymore for CentOS in case of upgrade from 10.3.

Change-Id: I9bedbf2821deb1411c038373c388e60df41e01b4

scripts/upgrade-utilities/galera-cluster-rolling-restart.yml

@@ -60,9 +60,9 @@
 
     - name: Check that WSREP is ready and Synced
       shell: "/usr/bin/mysqladmin extended-status | egrep '(wsrep_local_state_comment)'"
-      register: mysql_ready
+      register: mariadb_ready
       until:
-        - mysql_ready.rc == 0
-        - (mysql_ready.stdout).find("Synced") != -1
+        - mariadb_ready.rc == 0
+        - (mariadb_ready.stdout).find("Synced") != -1
       retries: 60
       delay: 1