Offer more general way to check domain level in replicainstall
Domain levels 0 and 1 use the same mechanism of checking domain
level correctness. Group them together and make it more general
should there be more domain levels in the future (although lets
hope there won't be).
Reviewed-By: Martin Basti <firstname.lastname@example.org>
Reviewed-By: Jan Cholasta <email@example.com>