mariadb: enable galera clustering by default

Kolla assumes mariadb 10.0 where galera stuff was always enabled. And
this works for CentOS and Ubuntu as they use 10.0 on x86-64.

Debian uses mariadb 10.1 by default (and CentOS on !x86-64 and
Ubuntu/aarch64) where galera stuff is disabled by default.

Closes-Bug: 1740060
Co-Authored-By: Xinliang Liu <xinliang.liu@linaro.org>
Change-Id: I8374ac2219ad7880970cd789727d01af7cac1077

ansible/roles/mariadb/templates/galera.cnf.j2

@@ -37,6 +37,7 @@ wsrep_sst_method=xtrabackup-v2
 wsrep_sst_auth={{ database_user }}:{{ database_password }}
 wsrep_slave_threads=4
 wsrep_notify_cmd=/usr/local/bin/wsrep-notify.sh
+wsrep_on = ON
 
 max_connections=10000