automember: fix automember to work with thin client
    
    Properly mark `cn` as primary key of `automember` object.
    
    This fixes automember crashing on output validation expecting primary key
    value of None.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>