Add k8s horizontal pod autoscaler

[ziegeer]

Investigate if sumo should use the k8s HPA. We currently have the cluster autoscaler so we can add new nodes if they're busy but it's unclear if we'd also need to add pods to take advantage of those new nodes or not.

[ziegeer]

As per the 2020-05-18 sumo incident report, we want to add HPA for increased reliability.