Mariabackup: read the same server groups

Currently only the xtrabackup and mysqld groups are read.
Its quite possible the user has put their server options in
a different group and mariadb will grab the wrong details.

Leverage off the server load_default_groups to populate this list.
This becomes particularly important with Galera SST scripts.

Despite the description and name, defaults_group has always been
an additional group. Change its help text to reflect this.