mariadb 10.2 was first released in April 2016 and OpenStack services
should be ready to use it by now, so lets switch to 10.2. For mariadb
10.2 we need to use xtrabackup-24. This version is not available in Leap
42.3 so we add an extra OBS repository for it.
Change-Id: I5f1aaf1f8608ad085acfebc8458910391f280193