Provides basic coverage for external trust feature.
Test cases:
1. verify an external trust with AD subdomain can be established
- verify only one trustdomain is listed
- verify subdomain users are resolvable
- verify trust can be deleted
2. verify non-external trust with AD subdomain cannot be established
3. verify an external trust with AD forest root domain can be established
- verify that even if AD subdomain is specified, it is not associated with the trust
- verify trust can be deleted
https://fedorahosted.org/freeipa/ticket/6093
Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>