Add a simple yamllint stage

This MR adds a new "yaml_lint" stage which unsurprisingly performs a yaml lint on any *.yml or *.yaml files that may be present

An example of a failed pipeline: package_alerts@5880f679 An example of a successful pipeline: reposync@b0a770fe

An improvement would be to install yamllint in the rpmci Dockerfiles, saving the requirement of doing it within the yaml_lint stage