This patch switches the code over to the payload style of callbacks [1]
for PORT ROUTER_GATEWAY events for those that are not using them yet.
The unit tests are also updated where needed to account for the
payload style callbacks and publish() method. Finally the patch
normalizes the passing of gateway IPs which are currently referred to
as 'gw_ips' and 'gateway_ips' depending on the event; now all events use
'gateway_ips'.
[1] https://docs.openstack.org/neutron-lib/latest/contributor/callbacks.html
Change-Id: Ibc255de79443e908cc3615a8e1cb108757f80011
eedbcdd03b