CLI cleanup

[acdha]

I was testing this locally and made several changes to make the process a bit nicer. Beyond some PEP-8 cleanup, here are the major changes:

• Use the AWS_DEFAULT_REGION environmental variable as the region if defined
• Use a mutually-exclusive option group so CLI users must explicitly select either a dry-run or deleting images
• The output is now more consistently formatted and has a nicer display of the image IDs which would be deleted