Automatic merge from submit-queue

Add ExternalName kube-dns e2e test

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

Built on top of and includes #30599 

See original issue at
https://github.com/kubernetes/kubernetes/issues/13748

Feature tracking at
https://github.com/kubernetes/features/issues/33

The e2e test is at least as comprehensive as the one for headless services (namely, only to some degree)

```release-note
Add ExternalName services as CNAME references to external ones
```