Add support for tenant-provided NSX gateways devices

Add support for tenant-provided NSX gateways devices

Add a new API resource specific to the NSX plugin for registering
tenant-owned NSX gateway devices with the NSX controller.
This API also allows tenants for managing gateway devices on the
NSX backend.

The behaviour of the net-gateway extension is mostly unchanged with
the only difference that newly created network gateways can now only
refer exlusively gateway devices registered using the API resource
introduced with this patch.

Implements blueprint nsx-remote-net-gw-integration

Change-Id: Ia2bdd0164498fe46a027b1d8f5a9d9f4e37558a4