Provides the framework for creating authentication plugins and using
them from a session object.
To allow this system to co-exist with the original client there is a bit
of a hack. The client object itself is now also an authentication
plugin, that supports the original client pattern. If a client is
created without a session object then that session object uses the
client as it's authentication plugin.
Change-Id: I682c8dcd3705148aaa804a91f4ed48a5b74bdc12
blueprint: auth-plugins
96267731ec