image pruning

Pruning images needs a little work.

• remove dependency on a local catalog of images - use refresh-releases code to get an in-memory assessment of what images are currently out there
• default for prune-amis (prune-images?) is to report what would be done. An additional flag must be provided to really do the pruning.
• don't report unknown images by default -- but the ability to do so is useful (finding images that weren't tagged properly)
• refresh-releases afterward to keep releases/<profile>.yaml updated with reality