Support L2GW plugin/agent

L2 Gateway (L2GW) is an API framework that offers bridging 2+
networks together to make them look as a single broadcast domain.
A typical use case is bridging the virtual with the physical networks.

Related-Prod: PROD-17614
Change-Id: I4101b94202f87da6f5ecdea75d38dde17f08ca99
14 files changed