IMAGE_URLS could be set both in localrc with customization or stackrc by
default. By setting DOWNLOAD_DEFAULT_IMAGES, user could choose to add
default images to IMAGE_URLS or overwrite them.
As uploading duplicate images will cause a "409 Conflict" error, a
duplicate detection will expose it earlier.
Care needs to be taken that you don't end up with a duplicate image, so
clean up Xen's README.
Depends-On: I6fbae12f950a03afab39f341132746d3db9f788c
Change-Id: I3ca4e576aa3fb8992c08ca44900a8c53dd4b4163
Closes-Bug: #1473432
a4693b5dea