Add option to delete cluster but retain S3 bucket

[cailinw]

Currently, deleting a cluster deletes the S3 bucket associated with it. This is because deleting a cluster should clean up all resources associated with the cluster. However, since the S3 bucket may contain data that existed before the cluster or has results written to it that the user would like to retain. The current solution is to copy the contents to preserve to a different S3 bucket before deleting the cluster. However, it would be nice to have an option to retain the S3 bucket when deleting a cluster.