Currently, Xtrabackup is used for database backups. However, Xtrabackup
is not compatible with MariaDB 10.3. This change switches to use
mariabackup [1], which is available in the mariadb image.

The documented full and incremental restore procedures have been
modified to use mariabackup, following [2] and [3].

[1] https://mariadb.com/kb/en/library/mariabackup-overview/
[2] https://mariadb.com/kb/en/library/full-backup-and-restore-with-mariabackup/
[3] https://mariadb.com/kb/en/library/incremental-backup-and-restore-with-mariabackup/

Change-Id: Id52b9b1f7b013277e401b1f6b8aed34473d2b2c4
Closes-Bug: #1843043
Depends-On: https://review.opendev.org/691290