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).
    
    https://fedorahosted.org/freeipa/ticket/6392
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>