Support AWS AZ balancing when terminating nodes

[mwhittington21]

When Escalator terminates nodes it should be possible to carefully select the terminated nodes so as to leave one node from each availability zone of the ASG. This is especially important when running nodes using PVCs and StatefulSets as they are essentially bound to nodes within their EBS volume's AZ.

If the minimum number of nodes in the ASG is less than the number of availability zones, just try to get as many AZs represented in there as possible.

[mwhittington21]

While we're at it, it would be great to include logging when scaling down of the AZ that each instance is in.