With the out of tree federation plugins going on extract the basic
federation workflow and required information that can be reused.

Change-Id: I6fdb3a5c6d9f3e1d6fa3425fd05809155effed1f

The basic Auth plugin for v3 tokens makes the assumption that you need
to pass in some AuthMethod objects. This works well for most auth types
where you want the plugin to construct the auth request for you.

In the case of federation though we want to be able to have a rescoping
plugin that will return an auth_ref and not take any auth_methods as
arguments.

Extract the most basic part of the Auth plugin into BaseAuth class that
Auth and federation plugins can both inherit from.

Change-Id: Ia8c8c614b8eb51170346ff5b1e20a1e7ebbb47de

The V3 authentication plugins file contained the existing plugins as
well as the base class. As we look to add new plugins it is simpler if
we break this file up.

Change-Id: I42b222a2012ea10491450d6b91c2008178dc7671

Some non-abstract methods only have docstring with no content,
this just add a default content to those function.

Change-Id: Idcf5b9f6ed766d3bc1541e158bdd8e58b06223e2

Generally we want people to use the existing plugins to manage their
authentication, however there are a number of existing services that
know how to work with an AccessInfo object directly and either cache it
or manipulate it manually.

Provide a simple Identity plugin that just takes an existing AccessInfo
and allows it to be used as an authentication plugin.

Change-Id: I388283c03a0a8a3d1afe43138eebbe5e66ca9102

The current scheme of having auth plugins only able to specify the
X-Auth-Token header via the get_token function is too limited for all
plugins. We need to allow both the case where the plugin wants to
control additional headers, or doesn't set the X-Auth-Token header at
all.

This deprecates the get_token interface in favour of the get_headers
interface. Whilst we should promote using get_headers it is likely that
plugins that only require setting the X-Auth-Token header will continue
to only support the get_token interface.

Change-Id: Ibd750d72acc3ba4fd8a880cad69173248ec4092f
blueprint: generic-plugins