Add plugin to support token-endpoint auth

The ksc auth plugins do not have support for the original
token-endpoint (aka token flow) auth where the user supplies
a token (possibly the Keystone admin_token) and an API endpoint.
This is used for bootstrapping Keystone but also has other uses
when a scoped user token is provided.

The api.auth:TokenEndpoint class is required to provide the
same interface methods so all of the special-case code branches
to support token-endpoint can be removed.

Some additional cleanups related to ClientManager and creating
the Compute client also were done to streamline using sessions.

Change-Id: I1a6059afa845a591eff92567ca346c09010a93af
Merge "Add plugin to support token-endpoint auth"