Create an Auth Plugin to pass to users

We pass the service catalog and token data through to consumers via
headers, with the auth plugin work we can create clients that are
authenticated with the incoming token and service catalog.

DocImpact: A new environment option is passed out of auth_token
middleware called `token_auth` which is an authentication plugin that
can be used with a session in a service.

Implements: blueprint create-auth-plugin
Change-Id: I549a421fc73f00126f6ac5f4b5911de2e32de825
Merge "Create an Auth Plugin to pass to users"