Automatic merge from submit-queue

make the system:authenticated group adder smarter

Fixes #42437 

This prevents the group adder from adding the system:authenticated group when:
 1. it's already in the list
 2. the user is system:anonymous
 3. system:unauthenticated is in the list

Smaller alternative to https://github.com/kubernetes/kubernetes/pull/42421 for 1.6.

@kubernetes/sig-auth-pr-reviews @enj @liggitt