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
61e7d1ebf1