Need a way to extend auth types

[Jamie Lennox]

Adding websso, kerberos and additional authentication mechanisms is going to get really messy and have dependency issues to have it all in tree. The basic password component of fetching an unscoped token can be easily replaced with any other plugin so we should make that extensible to allow other mechanisms to be developed outside the DOA repo

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/153910
Committed: https://git.openstack.org/cgit/openstack/django_openstack_auth/commit/?id=e6c25ad380dc1feb0ed121d088151d9fde8cacef
Submitter: Jenkins
Branch: master

commit e6c25ad380dc1feb0ed121d088151d9fde8cacef
Author: Jamie Lennox <email address hidden>
Date: Thu Feb 5 09:21:00 2015 +0000

    Create plugin model for DOA authentication

    With federated and kerberos logins coming we need an extensible way to
    specify additional ways to fetch an unscoped token from keystone.

    Create a plugin model that when authenticate is called a series of
    plugins can be queried for a token depending on the information
    provided.

    Closes-Bug: #1433389
    Change-Id: Ifbd7077173844a8eb3400799fd512b62a5dc7dcc