groupd: send and check version messages
    
    for better compatibility with and upgrades to cluster3 eventually.
    
    bz 457104
    
    - cluster3 nodes that are trying to detect old cluster2 nodes will be
      helped by the cluster2 groupd sending its version.
    - if someone adds a cluster2 node to a cluster3 cluster (which is not
      supposed to be done), detect the newer version and exit
    
    Signed-off-by: David Teigland <teigland@redhat.com>