Provide a conversion function for creating session

Session.construct will create a session based upon the kwargs that used
to be passed to a client __init__ function. This will allow clients an
easy path to providing compatibility with deprecated arguments.

Make use of the function throughout discovery. Discovery was initially
released prior to the session object being completed and was therefore
handled with the same arguments as a client. Instead we should use a
session object so use the conversion function to convert those kwargs
into a session object if one is not provided.

Change-Id: I8dc1e0810ea6ebc6ea648ec37d7881825c566676
Create Authentication Plugins

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
Merge "Provide a conversion function for creating session"
Merge "Create Authentication Plugins"