Automatic merge from submit-queue (batch tested with PRs 40796, 40878, 36033, 40838, 41210)

HPA v2 (API Changes)

**Release note**:
```release-note
Introduces an new alpha version of the Horizontal Pod Autoscaler including expanded support for specifying metrics.
```

Implements the API changes for kubernetes/features#117.

This implements #34754, which is the new design for the Horizontal Pod Autoscaler.  It includes improved support for custom metrics (and/or arbitrary metrics) as well as expanded support for resource metrics.  The new HPA object is introduces in the API group "autoscaling/v1alpha1".

Note that the improved custom metric support currently is limited to per pod metrics from Heapster -- attempting to use the new "object metrics" will simply result in an error.  This will change once #34586 is merged and implemented.