adtrust: support GSSAPI authentication to LDAP as Active Directory user
    
    In case an ID override was created for an Active Directory user in the
    default trust view, allow mapping the incoming GSSAPI authenticated
    connection to the ID override for this user.
    
    This allows to self-manage ID override parameters from the CLI, for
    example, SSH public keys or certificates. Admins can define what can be
    changed by the users via self-service permissions.
    
    Part of https://fedorahosted.org/freeipa/ticket/2149
    Part of https://fedorahosted.org/freeipa/ticket/3242
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>