This patches adds DB mixin classes for NSX extensions such as
network-gateway and qos-queue.
These extensions leverage the models defined in
openstack/neutron.
Import paths are adjusted accordingly throughout the repository,
including those for the api client modules and the plugin's
exception module.
Closes-Bug: #1417627
Change-Id: I0dfe2ecc591df6cbc95fd64483c2f0df56fb335c
ea75069fdc