disallow mod of topology segment nodes
    
    Mod of segment end will be disallowed in topology plugin.
    
    Reasoning (by Ludwig):  if we want to properly allow mods to change
    connectivity and endpoints, then we would need to check if the mod
    disconnects the topology, delete existing agreements, check if the new
    would be a duplicate and create new agmts. There could be some difficult
    scenarios, like having
      A <--> B <--> C <--> D,
    if you modify the segment B-C to A-D topology breaks and is then
    reconnected.
    
    part of: https://fedorahosted.org/freeipa/ticket/4302
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>