The yum-puppetlabs mirror exceeded its 100GB quota as of April 26.
Rather than increase the quota, start excluding packages for old
platforms we don't provide like RHEL5-6 and Fedora F20-27. We could
probably get even more aggressive with it, but this get the
utilization back under 50% which is plenty of headroom for now.
Change-Id: I9665b3a2a89f991f9433fe7f45bc1bb0e0c7632b