Listing of instances to be terminated before the prompt

Will list the EC2 instances before detroying the cluster.
This was added because it can be scary to destroy EC2
instances without knowing which one will be impacted.
Very minor - but it might be slightly nicer to write this using string interpolation and also just using two different print statements instead of putting a newline directly in there (since you'll get python's system-dependent line delimiter).
```
print "Are you sure you want to destroy the cluster %s?" % cluster_name
print "The following instances will be terminated:"
```