Extract the authentication code from a v3 client and move it to a series
of auth plugins. As v3 authentication can contain multiple
authentication methods this concept is represented by an AuthMethod. An
auth plugin then is provided with multiple mechanisms to authenticate
with.
There is also some helper class for the standard case where you only
need to authenticate with one method.
When a v3 client wants to do authentication it will create a new v3 auth
plugin, do the authentication and then take that result for the client
to use.
Change-Id: I5fa6a6e1c2e114e1428e35b723700c63a3cbed44
blueprint: auth-plugins
7f1881211d