Create shas for download assets

[rothgar]

When someone generates a downloadable image (e.g. iso) they want a way to verify their download with a sha. We should generate and publish a sha for assets that need to be manually downloaded in the image factory UI.

[smira]

Image Factory is naturally dynamic as it generates images on the fly. Some images are not reproducible (e.g. disk images).

ISOs should be fine, but due to the nature of the process we can't provide SHAs which are immutable.