Enumerating instances to be terminated before the prompting the users to continue.

When destroying a cluster, the user will be prompted for confirmation without first showing which instances will be terminated.

Pull Request: https://github.com/apache/spark/pull/270#issuecomment-46341975

This pull request will list the EC2 instances before destroying the cluster.
This was added because it can be scary to destroy EC2
instances without knowing which one will be affected.