Install cluster autoscaler

[pauldougan]

What

Install and configure the cluster autoscaler and configure appropriate limits

https://github.com/kubernetes/autoscaler/tree/master/cluster-autoscaler/cloudprovider/aws

Why

So we can automate scaling up/down of cluster (within reason)

If we don't do this

SREs will have to closely monitor capacity, or over provision worker nodes

It's done when

• The worker node group scales up when capacity is required
• The worker node group scales down when capacity is free (maybe - this is a harder problem I suspect)

[philandstuff]

Took a stab at this in #244

[chrisfarms]

resolved by #600 and friends