blueprint database-backup-recovery

Introduce a new option, mariadb_backup, which takes a backup of all
databases hosted in MariaDB.

Backups are performed using XtraBackup, the output of which is saved to
a dedicated Docker volume on the target host (which defaults to the
first node in the MariaDB cluster).

It supports either full (the default) or incremental backups.

Change-Id: Ied224c0d19b8734aa72092aaddd530155999dbc3