Add option to perform a drain before terminating a node

[awprice]

Add a option (in the nodegroup.yaml configuration file) to perform a drain before terminating an instance in a node group.

This option should be per node group, as some node groups may have jobs only vs some with services only.

[awprice]

We should be looking at performing a drain simulation to determine which nodes are the "easiest" to evict pods from and then terminate. This way we aren't wasting time trying to evict pods that can't easily be moved and try to terminate a different node.

Possibly use https://github.com/kubernetes/autoscaler/tree/master/cluster-autoscaler/simulator

[awprice]

I've created a seperate issue to look into different node selection methods for termination - #105