Automatic merge from submit-queue (batch tested with PRs 63011, 68089, 67944, 68132). If you want to cherry-pick this change to another branch, please follow the instructions here: https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md.

Mark HPA REST Clients Flag as Deprecated

This marks the option to switch to Heapster for the HPA as deprecated.
It'll be removed next release when Heapster is retired.

**Release note**:

```release-note
Using the Horizontal Pod Autoscaler with metrics from Heapster is now deprecated.
```