Automatic merge from submit-queue

Implement ExternalName in kube-dns

Part of the ongoing saga formerly known as https://github.com/kubernetes/features/issues/33

This is the prelude (first commit) in #30931, spawn into a separate PR to allow building a new kube-dns image before e2e tests (the rest of #30931) are updated.

ExternalName allows kubedns to return CNAME records for external
services. No proxying is involved.

cc @thockin