Fixes #3809 - Add rubocop to check Foreman code

This basically disables all the failing rules in Foreman and puts them in `.rubocop_todo.yml`. Over time, we'll need to address each cop by fixing the code or adding it to `.rubocop.yml` (if we don't want to enforce it). Then when all the cops are removed, we'll remove `.rubocop_todo.yml`.

Katello PR: https://github.com/Katello/katello/pull/4565

If you want to look up the cops to see what they do, visit http://rubydoc.info/gems/rubocop/0.24.1/frames