Store checksums of public images

Rewrites the public image handling in a more transactional way. Instead of writing one file at a time and throwing away the information about it, it writes a complete version with all images at a time and in the end writes a SHA512SUMS file including all checksums for this directory.

How to test

This change can't be checked in the pipeline right now. So use something like this to see what it does:

./bin/debian-cloud-images upload /tmp/t/*.json --provider test --storage out --debug --variant dev
tree out