Support for external extensions in Neutronclient

While neutronclient has support for showing extensions installed on the server, there's no explicit support in the client for extending it in a manner similar to novaclient. We would like to address this lack of support by providing the hooks necessary to add extensions to neutronclient via external packages as well as in the client library itself.

It appears Cliff itself can be used to facilitate this, so part of the blueprint will look at integrating that functionality into Neutronclient.