Draft: Replaced mirrors with vault for CentOS 8 image.

When building the CentOS 8 image provided from Pablo, I was running into an issue with the mirrors not being found. A quick search found this article: failed-metadata-repo-appstream-centos-8. The fix worked and I tested using our current document. I made this a draft to open the discussion about updating the system first. We have to change the mirrors to the vault mirrors to run dnf update first. That update then places more files with the original mirrors in the /etc/yum.repos.d/ directory. We would have to run the sed replace commands again for the additional files before moving to the dnf swap ... command. Since we are running the dnf distro-sync, is it necessary to do a full system update first?