rejigger make_new_ami.py

[dtenenba]

• Don't use scratch space on EBS volumes; instead 'bake in' scratch space with dm.basesize.
  (Note that get_jobdef_skeleton.py (and documentation) will also need to change.)
• Do everything in HSE account. Make it optional (not the default) to make AMIs public.
• Don't delete old AMIs automatically, indicate if there are still compute environments that use them.
• Automatically rebuild compute environments (and queues that use them?) when building a new
  AMI? What if jobs are running against an environment/queue?
• Add a command-line option to just output the latest ECS-optimized AMI ID.