Automatic merge from submit-queue
Audit webhook config for GCE
Add a `ADVANCED_AUDIT_BACKEND` (comma delimited list) environment variable to the GCE cluster config to select the audit backend, and add configuration for the webhook backend.
~~Based on the first commit from https://github.com/kubernetes/kubernetes/pull/46557~~
For kubernetes/features#22
Since this is GCE-only configuration plumbing, I think this should be exempt from code-freeze.
038d194723