Auth plugins

This supercedes #1358. 

Since some folks want a very simple default implementation for auth, and others want to be able to plug in a more full-featured implementation, I've focused on the problem of defining how Authorization plugs in.

In the process, I found I was unable to talk about that without also defining how Authentication and Group Membership should plug in.  

I've also removed the text from access.md about "userAccounts", since I no longer feel that is resource type has to be part of kubernetes.
Potentially yes.  We may want to spec this further by giving providers names that are disjoint with their type so that you can deal with test or multiple provider instance (two separate flows to google).  Haven't thought much about this.