init: Keep parent groups in addon comps environments
    
    The environment in comps for a variant can refer to groups in parent
    variant (either for addons, or because of other configuration). We
    should not remove the groups in this case.
    
    This requires changes in two places:
     * teaching `comps_filter` about groups that should not be removed
     * fixing writing comps so that it does not actually change the data as
       well
    
    JIRA: COMPOSE-2612
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>