Support AWS AZ balancing when terminating nodes

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.

Start by locating Escalator's node-termination path and the AWS ASG availability-zone data available there. Review how PVCs and StatefulSets depend on node AZs. Done means termination leaves at least one node per AZ when possible, or represents as many AZs as the minimum size allows.

Written by the indexing model from the issue text.