Document the auth plugins that are loadable by name
Document the auth plugins that are loadable by name

These are the plugins that will be available for use by auth_token
middleware and other services as they adopt sessions.

There needs to be a good way to document the config parameters that each
plugin exposes, because these are not necessarily the same as what are
available via __init__.

Change-Id: I4ad8ea89381be7571c8a30a6fcd9162c259eb581

sdpp/python-keystoneclient,darren-wang/ksc,sdpp/python-keystoneclient,klmitch/python-keystoneclient,ging/python-keystoneclient,klmitch/python-keystoneclient,ging/python-keystoneclient,magic0704/python-keystoneclient,Mercador/python-keystoneclient,darren-wang/ksc,Mercador/python-keystoneclient,magic0704/python-keystoneclient