add a setting of a bonding i/f.
        'add' method takes the corresponding args in this order.

        ========= =====================================================
        Attribute Description
        ========= =====================================================
        dpid      datapath id.

        ports     a list of integer values that means the ports face
                  with the slave i/fs.
        ========= =====================================================

        if you want to use multi LAG, call 'add' method more than once.

def add(self, dpid, ports):
        """add a setting of a bonding i/f.
        'add' method takes the corresponding args in this order.

        ========= =====================================================
        Attribute Description
        ========= =====================================================
        dpid      datapath id.

        ports     a list of integer values that means the ports face
                  with the slave i/fs.
        ========= =====================================================

        if you want to use multi LAG, call 'add' method more than once.
        """
        assert isinstance(ports, list)
        assert len(ports) >= 2
        ifs = {}
        for port in ports:
            ifs[port] = {'enabled': False, 'timeout': 0}
        bond = {dpid: ifs}
        self._bonds.append(bond)