Check for the presence of the forest root DNS domain of the AD realm
among the IPA realm domains prior to esablishing the trust.
This prevents creation of a failing setup, as trusts would not work
properly in this case.
https://fedorahosted.org/freeipa/ticket/4799
Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>