Add Quantum support for NVP Layer-2 gateways

Blueprint nvp-nwgw-api

This patch adds an API extension, the relevant DB logic, and the NVP
plugin logic for managing a NVP-specific feature, Layer-2 Network
Gateway, through the Quantum API.
The proposed extension is meant to be used with the NVP plugin only.

Change-Id: I73a8f1782c345ca7f6dec2db36ba6f9299b30d04