The new build pipeline compresses images already but uploaded both the
compressed and uncompressed files because the whole build folder gets
uploaded.
Add a new flag "--only_store_compressed" to the image generation which
deletes the uncompressed file after compression is done. Uncompressed
images are still supported if specified in the flag
"image_compression_formats".
Closes https://github.com/flatcar-linux/Flatcar/issues/793