Automatic merge from submit-queue

HPA Status Conditions

This PR introduces conditions to the status of the HorizontalPodAutoscaler (in autoscaling/v2alpha1).  
The conditions whether or not the autoscaler is actively scaling, and why.  This gives greater visibility
into the *current* status of the autoscaler, similarly to how conditions work for pods, nodes, etc.

`kubectl describe` has been updated to the display the conditions affecting a given HPA.

Implements kubernetes/features#264 (alpha in 1.7)

**Release note**:
```release-note
Introduces status conditions to the HorizontalPodAutoscaler in autoscaling/v2alpha1, indicating the current status of a given HorizontalPodAutoscaler, and why it is or is not scaling.
```