topology: restrict direction changes
    
    topology plugin doesn't properly handle:
    - creation of segment with direction 'none' and then upgrade to other
      direction
    - downgrade of direction
    
    These situations are now forbidden in API.
    
    part of: https://fedorahosted.org/freeipa/ticket/4302
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>