Add SSDs to block device mapping

On `m3.2xlarge` instances the 2x80GB SSDs are inaccessible if not added to the block device mapping when the instance is created. They work when added with this patch. I have not tested this with other instance types, and I do not know much about this script and EC2 deployment in general. Maybe this code needs to depend on the instance type.

The requirement for this mapping is described in the AWS docs at:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/InstanceStorage.html#InstanceStore_UsageScenarios
This format syntax won't work in Python 2.6 unfortunately; use `"emphemeral%d" % i`