VMWare-NSXv: VMWare NSXv database models

For Kilo, the vendor-specific code should be moved to stackforge repo,
excluding the database models (https://review.openstack.org/#/c/134680/).
This patch adds the database model for VMWare NSXv plugin from
stackforge/vmware-nsx repo.

Partially-Implements: blueprint vmware-nsx-v

Change-Id: I17cbc1ba59131bd3bdefa232800842a9e739945f