Auth plugin valid arguments do something more

There was some discussion on http://eavesdrop.openstack.org/meetings/python_openstacksdk/2014/python_openstacksdk.2014-10-07-19.00.log.txt about doing something more about valid arguments, validation, etc I think the idea was to have the plugin have some sort of validate_arguments class method or something of the sort.

Origiinally, I created a bug to track this, but this is really a feature: https://bugs.launchpad.net/unifiedsdk/+bug/1377424