Add mysql version selector to galera pillars

PROD-36708

Change-Id: I756a2b0504492e76fc66623ba9d9eef22fce3928
diff --git a/defaults/openstack/init.yml b/defaults/openstack/init.yml
index b01094a..15a718f 100644
--- a/defaults/openstack/init.yml
+++ b/defaults/openstack/init.yml
@@ -27,6 +27,7 @@
     # RabbitMQ
     rabbitmq_upgrade_enabled: ${_param:openstack_upgrade_enabled}
     # Galera
+    galera_mysql_version: '5.6'
     galera_upgrade_enabled: ${_param:openstack_upgrade_enabled}
     # Openstack memcache
     openstack_memcached_server_bind_address: 0.0.0.0