Return minions which match the special list-only groups defined by
        ssh_list_nodegroups

def ret_nodegroup_minions(self):
        '''
        Return minions which match the special list-only groups defined by
        ssh_list_nodegroups
        '''
        nodegroup = __opts__.get('ssh_list_nodegroups', {}).get(self.tgt, [])
        nodegroup = _tgt_set(nodegroup)
        return self._ret_minions(nodegroup.intersection)