Convert TaaS to a Neutron L2 extension

[Reedip]

Currently Tap-as-a-Service is a separate agent.
However, it can be modulated and converted to a more appropriate L2 extension for Neutron which can be extended and used by third party vendors.

Update etherpad: https://etherpad.openstack.org/p/l2-agent-extensions-
api-expansion

[YAMAMOTO Takashi (yamamoto)]

relevant etherpad: https://etherpad.openstack.org/p/l2-agent-extensions-api-expansion

[Reedip (reedip-banerjee-deactivatedaccount)]

Review reference: https://review.openstack.org/#/c/401258/

[Reedip (reedip-banerjee-deactivatedaccount)]

Committed in https://review.openstack.org/#/c/480031/

[OpenStack Infra (hudson-openstack)]

Change abandoned by Reedip (<email address hidden>) on branch: master
Review: https://review.openstack.org/401258
Reason: Already merged

[YAMAMOTO Takashi (yamamoto)]

https://review.openstack.org/#/c/480031/