Add third-party support for Quantum NVP plugin

This patch leverages the third-party mechanism provided by DevStack
to customize the configuration of DevStack instances when working
with the Quantum NVP plugin. This is useful in dev/test scenarios,
where connectivity between the DevStack VM and the NVP Gateway is
required.

Supports blueprint nvp-third-party-support

Change-Id: I3f5afa5de1219f491e37c8b9b28370855d6b017c
Merge "Add third-party support for Quantum NVP plugin"