[OVN] GW rescheduling logic is broken

When a Chassis event happens in the SB database, we attempt to reschedule any possible unhosted gateways [0] *always* due to a problem with the existing logic:

    def get_unhosted_gateways(self, port_physnet_dict, chassis_physnets,
                              gw_chassis):
        unhosted_gateways = []
        for lrp in self._tables['Logical_Router_Port'].rows.values():
            if not lrp.name.startswith('lrp-'):
                continue
            physnet = port_physnet_dict.get(lrp.name[len('lrp-'):])
            chassis_list = self._get_logical_router_port_gateway_chassis(lrp)
            is_max_gw_reached = len(chassis_list) < ovn_const.MAX_GW_CHASSIS
            for chassis_name, prio in chassis_list:
                # TODO(azbiswas): Handle the case when a chassis is no
                # longer valid. This may involve moving conntrack states,
                # so it needs to discussed in the OVN community first.
                if is_max_gw_reached or utils.is_gateway_chassis_invalid(
                        chassis_name, gw_chassis, physnet, chassis_physnets):
                    unhosted_gateways.append(lrp.name)
        return unhosted_gateways

1) is_max_gw_reached is always going to be True (as normally the possible candidates are less than the maximum)

2) unhosted_gateways.append(lrp.name) is executed inside a loop where lrp doesn't change meaning that if there's 3 candidates in the chassis_list, lrp.name is added 3 times to the list!!!

3) Later on, in the caller, we're iterating over the returned list [1] so as it has all the LRPs N times (N being the names of gw chassis), it will do a lot of extra and unnecessary work.

This is almost harmless in the sense that it's not breaking any functionality but it creates unnecessary updates on the logical router port:

2020-01-31 15:54:04.669 37 DEBUG ovsdbapp.backend.ovs_idl.transaction [-] Running txn n=1 command(idx=0): UpdateLRouterPortCommand(name=lrp-93b49ece-2dbc-4fcc-84cb-e7afd482a12e, columns={'gateway_chassis': ['0444b1f1-e9a9-4a73-ba78-997c87e61795', '43d98571-ccd6-48ce-bf4f-08f24aeed522', 'fe8f9887-27ef-4724-8cfc-50ec6e3d4a98']}, if_exists=True) do_commit /usr/lib/python3.6/site-packages/ovsdbapp/backend/ovs_idl/transaction.py:84
2020-01-31 15:54:04.670 37 DEBUG ovsdbapp.backend.ovs_idl.transaction [-] Transaction caused no change do_commit /usr/lib/python3.6/site-packages/ovsdbapp/backend/ovs_idl/transaction.py:121

[0] https://github.com/openstack/neutron/blob/4689564fa29915b042547bdeb3dcb44bca54e20c/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/impl_idl_ovn.py#L449
[1] https://github.com/openstack/neutron/blob/858d7f33950a80c73501377a4b2cd36b915d0f40/neutron/services/ovn_l3/plugin.py#L324