Add Rubocop

I left out the spec files because serverspec does not work well with rubocop. I initially tried to make it work but it ends up making the spec files unreadable.

This is to get the cookbook ready to work on the multi-host haproxy config recipe.
So a cleaner way to do this can be found [here](https://github.com/osuosl-cookbooks/proj-phpbb/blob/master/recipes/haproxy.rb#L33)