Automatic merge from submit-queue

K8s 1.5 keeps container-vm as default node image on GCE

There is a concern that some GCE users may be running automation that
(a) turns up ephemeral clusters and (b) always uses the latest K8s
release. If any of these workloads fall outside the set supported on
GCI, cutting the release will break the automation. We are therefore
delaying this change until we have provided sufficient warning.

```release-note
K8s 1.5 keeps container-vm as the default node image on GCE for backwards compatibility reasons. Please beware that container-vm is officially deprecated and you should replace it with GCI if at all possible. You can review the migration guide here for more detail: https://cloud.google.com/container-engine/docs/node-image-migration
```

/cc @aronchick @vishh @roberthbailey