When `trust-add` is supplied AD domain admin name without realm component, the code appends the uppercased AD forest root domain name to construct the full principal. This can cause authentication error, however, when external trust with non-root domain is requested. We should instead use the supplied DNS domain name (if valid) as a realm component. https://fedorahosted.org/freeipa/ticket/6277 Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>