bacula/backups: Use defaults-extra-file instead

Use --defaults-extra-file instead of --defaults-file. Also use
/usr/local/bin/mysqldump and /usr/local/bin/mysql for the mariadb
backups. Rely on the symlink being correct instead of relying in some
autodetection/hardcoding of the actual MariaDB installation and/or
system packages

Change-Id: I026b7e1dd2f1817710b3a481c8a1296ad7a65ab8