`remote-prune` should query EC2 for AMIs

[miabbott]

Instead of relying on AMIs listed in the build metadata, we should query EC2 directly for the list of AMIs and cross-check. That way, if we're interrupted or they're desynchronized somehow, we don't leak images.

(Taken from @jlebon's comment https://github.com/coreos/coreos-assembler/pull/839#issuecomment-548556519)