Replace Percona XtraBackup with Mariabackup

We use MariaDB 10.3 in RHEL-based and Debian images. However, it is not supported by Percona XtraBackup (and it is unlikely it will ever be). MariaDB offers almost-drop-in replacement, Mariabackup, since 10.1, however it is not shipped with Ubuntu bionic (even though it officially has 10.1). The proposed path is to again use the upstream repos and Mariabackup instead of Percona XtraBackup. This also affects kolla-ansible.