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
Move plugin stuff to clientmanager

The OSC plugins work by adding an object as an attribute to a
ClientManager instance.  The initialization and management of
thos plugins belongs in clientmanager.py.

At this point the only part not moved is the API version dict
initialization bcause the timing and connection to the
CommandManager initialization.  It gets refactored anyway when
API discovery becomes operational.

Change-Id: If9cb9a0c45a3a577082a5cdbb793769211f20ebb
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 "Move plugin stuff to clientmanager"
Merge "use jsonutils in oslo.serialization instead of keystoneclient"
Merge "Add plugin to support token-endpoint auth"