Collect auth args in a more oganized way

[Terry Howe]

Use a special class or at least list of valid arguments to collect authorization arguments. I think this is taken care of in the current pipe, but I'd like to track this here. Doug's first comment in what is throw away code:

https://review.openstack.org/#/c/118679/7/openstack/auth/identity/authenticator.py

    The rest of this looks like it wants to be a loop.
      args.update( (n, v) for (n, v) in [('project_name', project_name),...] if v )
    It might also just make sense to define a little class to be a container for auth args. It wouldn't do anything except define which
    args are valid anywhere at all, and would let us pass a single thing around instead of a bunch of **kwds.

Later he suggests using a special class:

https://review.openstack.org/#/c/119628/4/openstack/auth/identity/authenticator.py

     Referring back to my comment on the base changeset, we could remove this loop if we defined an AuthArgumentContainer class
     that always held all of the arguments. I'm a little worried we can end up with a call to a plugin here that doesn't pass a required
     argument, though I guess we can mitigate that risk by always having the plugins define default values for their arguments.

This authenticator.py method is throw away code.