[Reviewer: Andy] Improve support for multiple regions

Andy,

Please can you review my fixes to improve support for multiple regions.  This involves
- selecting the correct image, based on region
- providing the region when doing deletion and cleanup.

Note that I don't really like the `:default` logic - it seems that Ruby dictionaries have a `.default` property that can be used.  Unfortunately, there doesn't seem to be a concise way of setting this, so I've gone for this approach instead.

Matt