Automatic merge from submit-queue
Add Service type "ExternalName" which results in CNAME DNS
ExternalName allows kubedns to return CNAME records for external
services. No proxying is involved.
First step for https://github.com/kubernetes/features/issues/33
See original issue at
https://github.com/kubernetes/kubernetes/issues/13748
No release note yet, that will come with the kubedns change.
```release-note
NONE
```