qdiskd: restrict master_wins to 2 node cluster
    
    given enough mingling of cluster.conf it was possible to
    break quorum rule #1: there is only one quorum in a cluster at
    any given time.
    
    this change restricts master_wins to 2 node cluster only
    and provides extra feedback to the user (via logging) on why
    the mode is disabled.
    
    Resolves: rhbz#838047
    
    Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>
    Reviewed-by: Lon Hohberger <lhh@redhat.com>