Code review comment for lp://staging/~ltrager/maas-images/image_index

Thanks for the review. I've removed the old code as suggested and added support for upstream release versions. I'm ignoring old revisions formats(e.g 20141129) since we aren't building them anyway. While you are correct CentOS 6 and 7 may use the same revision they come from different files[1, 2] which are processed separately.

[1] https://cloud.centos.org/centos/7/images/image-index
[2] https://cloud.centos.org/centos/6/images/image-index