Automatic merge from submit-queue (batch tested with PRs 51603, 51653)

Graduate metrics/v1alpha1 to v1beta1

This introduces v1beta1 of the resource metrics API, previously in alpha.
The v1alpha1 version remains for compatibility with the Heapster legacy version
of the resource metrics API, which is compatible with the v1alpha1 version.  It also
renames the v1beta1 version to `resource-metrics.metrics.k8s.io`.

The HPA controller's REST clients (but not the legacy client) have been migrated as well.

Part of kubernetes/features#118.

```release-note
Migrate the metrics/v1alpha1 API to metrics/v1beta1.  The HorizontalPodAutoscaler
controller REST client now uses that version.  For v1beta1, the API is now known as
resource-metrics.metrics.k8s.io.
```