Use new ksc features in User Token Plugin

When the user token plugin was created some problems were noticed.

Firstly the way the auth_ref was being constructed meant that it would
try and retrieve the token from the body of the message which wouldn't
work for v3 or PKI tokens. To overcome this we stored the token data in
the plugin and added the ability to override the token data in the
auth_ref.

Secondly that there was no way to signal that this plugin couldn't be
reauthenticated and it would therefore retry unauthenticated requests
that we knew wouldn't work.

Both of these issues were addressed in keystoneclient and they should be
updated to work correctly in auth_token middleware.

Change-Id: Ib1c772e55f7c7b622d0c2a55d87d77d2dc30d4bb
Allow loading other auth methods in auth_token

Add handlers for loading user specified authentication plugins from the
config file. If an auth plugin is not specified then we fall back to
using the older legacy options.

Change-Id: I4d09a30c0163106dab52062fab6000aaec0efb5d
Closes-Bug: #1372142
Blueprint: pluggable-auth
Merge "Allow loading other auth methods in auth_token"
Merge "Use new ksc features in User Token Plugin"