Adding mechanism driver in ML2 plugin for Nuage Networks

This patchset introduces basic ml2 driver for nuage.
In Juno release, mechanism driver will support basic
L2 functionality as a stepping stone to enhance
it in later releases.

Implements blueprint: ml2-mech-driver-nuage
Change-Id: Idae4f88f3d21526f377ec0f81377cb90b9fc14e4
Merge "Adding mechanism driver in ML2 plugin for Nuage Networks"