Optinally enable conversion of images to raw format when uploading
images to Glance. Only supported when "image_conversion" is enabled in
Glance.
This is an equivalent change to the one in simplestreams:
https://git.launchpad.net/simplestreams/commit/?id=cbc0ac9
Closes-Bug: #1889729
Change-Id: Id15fab364c27242347c520b959065c490e4da6b6