Automatic merge from submit-queue

Prevent protobuf storage with etcd2

Prevents accidentally storing protobuf content in etcd2 when upgrading to 1.6

c.f. https://github.com/kubernetes/kubernetes/issues/42976#issuecomment-286537139

```release-note
if kube-apiserver is started with `--storage-backend=etcd2`, the media type `application/json` is used.
```