Allow instantiation with just token and endpoint

Pretty much all the other openstack clients (keystoneclient, glanceclient, swiftclient) have an official way to create a client by passing in solely a pre-authorized token and the appropriate endpoint (aka auth_token and management_url).

horizon even hacks around the fact that novaclient doesn't support this already by manually setting the auth_token and management-url on the client after creating it but before making any calls so it skips the "authenticate" step.

There's no reason not to make this a supported feature.