While cluster creation, if we specify boot volume types as st1 or sc1 for nodes, the cluster creation continues but doesn't create desired nodes as st1 and sc1 are not supported by AWS for creating boot volumes.

In such cases, a friendly warning message mentioning this fact would be nice so that a user can understand that these volume types cannot be used as boot volumes and users won't keep wondering why kops is not able to create required nodes.

Same goes while updating instance groups' volume types from gp2 -> st1. kops continues with this change and even shows something like this:

Will modify resources:
  LaunchConfiguration/nodes.xyz.k8s.local
  	RootVolumeSize      	 128 -> 500
  	RootVolumeType      	 gp2 -> st1

And if at this point, a user is unaware of the above-mentioned fact, he'll likely continue with kops update cluster --yes which also wouldn't show anything suspicious. The error comes only when we do a rolling update.

Instead if we can have a warning at the very beginning, it will be very helpful and will save some time as well.

For more info, see #4256