Unpack AWS reservations while waiting to terminate (#4012)

Previously calculation of the number of instances that have been
terminated assumed all instances were in the first reservation returned
by AWS.  If this is not the case the calculated number of instances
terminated never reaches the number of instances and the module always
times out. By unpacking the instances we get an accurate number and the
module correctly exits.

lib/ansible/modules/cloud/amazon/ec2.py

@@ -1233,7 +1233,7 @@ def terminate_instances(module, ec2, instance_ids):
                 instance_ids=terminated_instance_ids, \
                 filters={'instance-state-name':'terminated'})
             try:
-                num_terminated = len(response.pop().instances)
+                num_terminated = sum([len(res.instances) for res in response])
             except Exception as e:
                 # got a bad response of some sort, possibly due to
                 # stale/cached data. Wait a second and then try again