Refactoring cleanup for L3 agent callbacks

Refactoring cleanup for L3 agent callbacks

This commit completes the refactoring of the L3 agent callback mechanism.
The goal here is to also use the neutron/callbacks/ mechanism for L3 agent
notifications, instead of have two mechanisms.

[1] modified the L3 agent to send notifiactions for router create, udpate,
and delete events, using the neutron/callbacks/ mechanism.

[2] modified VPN to use this new mechanism, instead of the L3EventObservers
mechanism. Note:

[3] modified FW repo to no longer depended on the L3EventObserver and
related objects (it doesn't currently use the event notifications).

This commit removes the notifications for the L3EventObservers mechanism,
removed the related modules and tests, and adds in tests to verify that the
new notifications are called for the different events.

Once [1] and [2] are upstreamed, this commit can proceed.

Refs:
[1] https://review.openstack.org/#/c/164466/
[2] https://review.openstack.org/#/c/165226/
[3] https://review.openstack.org/#/c/167275/

Change-Id: I7c4b4ea5f9fb19abb812665cdae5fb70c84fe3ec
Depends-On: If5040a827a6903cc7cb5e59cdb7fb95f61b13d47
Closes-Bug: #1433552