Supporting Net-Partition as extension for Nuage plugin

A net-partition provides administrative separation between groups of tenant
routers/networks/subnets. Within a net-partition, subnets can be shared, and
VMs can connect multiple vNICs to subnets within the net-partition.
For security and isolation between organizations, VMs may not connect
VNICs to subnets within more than one net-partition.

This work is in accordance with bp/nuage-networks-plugin which is
under review: https://review.openstack.org/#/c/67272/

Change-Id: Ia9b0304f714c65548cd6b8ce6ff74a3075c4e673
Implements: blueprint nuage-net-partition-extension
Merge "Supporting Net-Partition as extension for Nuage plugin"