Supporting Nuage Networks' Plugin through devstack

Nuage networks' plugin specific configuration setting file for devstack

Change-Id: I936f87b8fbc6f90130514b2fc0d111eab861da7c
Implements: blueprint nuage-networks-plugin
Merge "Supporting Nuage Networks' Plugin through devstack"
Merge branch 'master' of github.com:openstack-dev/devstack