Align volume delete on terminate with AWS defaults

This pull request changes the default delete_on_termination flag to True, as it is in AWS.  It also updates register_image to allow users to make an explicit choice of whether to delete on termination for images created with the snapshot_id arg.

I made this change after discovering that our new image creation process was leaving volumes behind.  When we discovered this, we had about 4.5TB of volumes that we had generated over the past few weeks from launching and terminating autoscaled instances.

This was unexpected for us, as the AWS default is to delete volumes on termination unless explicitly told not to.