Add security groups events

Add security groups events

ML2 mech drivers have no direct exposure to security groups,
and they can only infer them from the associated network/ports.
This is problematic as agentless ML2 mech drivers have no way of
intercepting securitygroups events and propagate the information
to their backend, or more generally, react to them.

This patch leverages the callback registry to dispatch such events
so that interested ML2 mech drivers (or any interested party like
service plugins) can be notified and react accordingly.

This patch addresses create/update/delete of security groups and
create/delete of security groups rules. Other events may be added
over time, if need be.

This patch is only about emitting the events. The actual subscription
and implementation of the event handlers will have to take place where
deemed appropriate.

Closes-bug: #1444112

Change-Id: Ifa1d7ee9c967576f824f1129dd68e6e3abd48f5c