Downloadable images are enabled when key image-url-list (described in
instance-kvm-input-schema.json) is present.

Images are downloaded outside of partition processing, as this can take a lot
of time by template/image-download-controller.py

Configuration is checked and cleaned up by
template/image-download-config-creator.py

Promises are used for:

 * checking if the current configuration from the request has been processed
 * checking status of configuration generation, image download process and
   checksum validity

Details about errors are exposed by using monitor stack provided HTTP server,
so that user is able to take informed decision about how to fix the problem.