Engine: rubocop enabled

We've re-enabled rubocop in the engine branch of Katello and once this PR is merged (https://github.com/Katello/katello/pull/3456), Jenkins should be green.

If you have any Katello PRs going to the engine branch, please rebase and then have Jenkins rerun them. Also, please validate that the Jenkins rubocop job gets run and is green.

Thanks!

David