Capture instance images

This adds the sub commands ``gonzo image-create`` and ``gonzo image-delete`` as well as making the option ``gonzo instance-launch --image-name`` available.

The end goal for this functionality is to enable snapshots to be created automatically after a stack's launch has completed. Something like ``gonzo stack-launch --create-instance-images --delete-upon-completion``, which would create snapshots for each instance resource and then delete the stack. These snapshots could then be utilised by a "sister" template which will launch far quicker.