HPA Controller: Use Custom Metrics API

This commit switches over the HPA controller to use the custom metrics
API.  It also converts the HPA controller to use the generated client
in k8s.io/metrics for the resource metrics API.

In order to enable support, you must enable
`--horizontal-pod-autoscaler-use-rest-clients` on the
controller-manager, which will switch the HPA controller's MetricsClient
implementation over to use the standard rest clients for both custom
metrics and resource metrics.  This requires that at the least resource
metrics API is registered with kube-aggregator, and that the controller
manager is pointed at kube-aggregator.  For this to work, Heapster
must be serving the new-style API server (`--api-server=true`).

hack/verify-flags/known-flags.txt

@@ -691,3 +691,4 @@ www-prefix
 zone-id
 zone-name
 
+horizontal-pod-autoscaler-use-rest-clients