REP-996 Refactor OpenStack Identity V3 Filter

I've opted out of changing the header code. I couldn't come up with a solution I really liked. If someone else wants to pick up that task, I'd love to see what you can come up with! That task can be done at a later time if we want to put it off, or we can abandon it. Or I can do the work if someone wants to provide the design.

Apart from that, comment away!