When DNS is already installed somewhere in topology we should not check for
zone overlap because it would always say that we are overlapping our own domain.
ipa-replica-install already does that but ipa-dns-install did not.
https://fedorahosted.org/freeipa/ticket/5564
Reviewed-By: Petr Spacek <pspacek@redhat.com>
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>