download: Reorder to have CentOS first
download: Reorder to have CentOS first

This is a tricky balance but I think we should point people at more
production configurations first.  Also, we keep having embarassing
bugs in Fedora.  Fedora and CAHC should more be development options.

For now I'm just doing a reordering, but I plan to clean this up more.

nzwulfin/atomic-site,nzwulfin/atomic-site,bexelbie/atomic-site,nzwulfin/atomic-site,bexelbie/atomic-site,bexelbie/atomic-site,nzwulfin/atomic-site,bexelbie/atomic-site,nzwulfin/atomic-site,bexelbie/atomic-site