ipaserver/dcerpc.py: make PDC discovery more robust
    
    Certain operations against AD domain controller can only be done if its
    FSMO role is primary domain controller. We need to use writable DC and
    PDC when creating trust and updating name suffix routing information.
    
    https://fedorahosted.org/freeipa/ticket/4479
    
    Reviewed-By: Sumit Bose <sbose@redhat.com>