CI should test validity of plugins/config/repository.yaml

[scpeters]

I broke the syntax of plugins/config/repository.yaml in #16, but CI didn't complain. I started noticing CI builds failing with the following message:

mapping values are not allowed here
  in "/var/lib/jenkins/workspace/ignition_fuel-tools2-install-pkg-bionic-amd64/gzdev/plugins/config/repository.yaml", line 36, column 17

I fixed it in a direct commit in 8aece40, but I think we should validate this yaml file in CI.