trusts: Add support for an external trust to Active Directory domain
    
    External trust is a trust that can be created between Active Directory
    domains that are in different forests or between an Active Directory
    domain. Since FreeIPA does not support non-Kerberos means of
    communication, external trust to Windows NT 4.0 or earlier domains is
    not supported.
    
    The external trust is not transitive and can be established to any
    domain in another forest. This means no access beyond the external
    domain is possible via the trust link.
    
    Resolves: https://fedorahosted.org/freeipa/ticket/5743
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>