SPARK-2166 - 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.

Author: Jean-Martin Archer <jeanmartin.archer@pulseenergy.com>

This patch had conflicts when merged, resolved by
Committer: Patrick Wendell <pwendell@gmail.com>

Closes #270 from j-martin/master and squashes the following commits:

826455f [Jean-Martin Archer] [SPARK-2611] Implementing recommendations
27b0a36 [Jean-Martin Archer] 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.

ec2/spark_ec2.py

@@ -770,12 +770,16 @@ def real_main():
         setup_cluster(conn, master_nodes, slave_nodes, opts, True)
 
     elif action == "destroy":
-        response = raw_input("Are you sure you want to destroy the cluster " +
-                             cluster_name + "?\nALL DATA ON ALL NODES WILL BE LOST!!\n" +
-                             "Destroy cluster " + cluster_name + " (y/N): ")
+        print "Are you sure you want to destroy the cluster %s?" % cluster_name
+        print "The following instances will be terminated:"
+        (master_nodes, slave_nodes) = get_existing_cluster(
+            conn, opts, cluster_name, die_on_error=False)
+        for inst in master_nodes + slave_nodes:
+            print "> %s" % inst.public_dns_name
+
+        msg = "ALL DATA ON ALL NODES WILL BE LOST!!\nDestroy cluster %s (y/N): " % cluster_name
+        response = raw_input(msg)
         if response == "y":
-            (master_nodes, slave_nodes) = get_existing_cluster(
-                conn, opts, cluster_name, die_on_error=False)
             print "Terminating master..."
             for inst in master_nodes:
                 inst.terminate()