VMWare-NSXv: VMWare NSXv extensions

VMWare-NSXv: VMWare NSXv extensions

For Kilo, the vendor-specific code should be moved to stackforge repo,
excluding the extensions (https://review.openstack.org/#/c/134680/).

This patch adds the extensions VMWare NSXv plugin from
stackforge/vmware-nsx repo.

There are 2 new extensions:
1. advanced_service_providers - this is required to enabled the NSXv to provide
                                metadata support (there is no metadata agent)
2. vnic_index - the security group support for the NSXv requires the vNIC index

Change-Id: I8c7aa95db1caf020afaa217d9c543b9031fab371
Partially-Implements: blueprint vmware-nsx-v