CentOS 7 is going EOL in the near future. Due to a lack of demand/need
within OpenDev and the upcoming EOL OpenDev has decided to drop CentOS 7
test nodes entirely. This will free up room for OpenDev to build and
mirror distro releases that are more relevant today. In preparation for
this removal we drop the zuul-jobs CentOS 7 testing.
Change-Id: Id49d28dd426ecbb3470e234d3b67875a6b097112
f2f5220561