Standardize session and auth plugin CLI loading parameters

In the time between code review for loading Session from cli and auth plugins for cli the function names changed.

This means we now have the case where to load both from argparse looks like:

session.Session.register_cli_options(parser)
auth.register_argparse_arguments(parser)

plugin = auth.load_from_argparse_arguments(parser)
sess = session.Session.load_from_cli_options(parser, auth=plugin)

whereas in the CONF case both session and auth use the same register_conf_options and load_from_conf_options.

This always makes me twitch a little when using it, and is not an obvious experience.

Extract an Abstract Base Class with the methods we provide and standarize these options.

I can see this being useful in the future to provide a full keystoneclient.Client.load_from_argparse_arguments or at least Adapter.