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
use jsonutils in oslo.serialization instead of keystoneclient

keystoneclient/openstack/common/jsonutils.py is removed in this patch
https://review.openstack.org/#/c/128454/
Now, we should use jsonutils in oslo.serialization package.

Change-Id: I7c8e8e6d5dffa85244368fd578616c9b19f4fd21
Merge "use jsonutils in oslo.serialization instead of keystoneclient"
Merge "Add plugin to support token-endpoint auth"