trusts: Check for AD root domain among our trusted domains
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>