This patch switches callbacks over to the payload object style events
[1] for BEFORE_READ based notifications. To do so an EventPayload object
is used with the publish() method to pass along the API related data.
In addition a few UTs are updated to work with the changes.
NeutronLibImpact
[1] https://docs.openstack.org/neutron-lib/latest/contributor/callbacks.html#event-payloads
Change-Id: Iff3e96c56867b4bf4272fed676f39cd6796d756c