Add repo switch, and add chefspec tests

In my company, i have no right to contact the outside world. I need to use internal repository to install varnish. So i add a switch to bypass repo installation, which is managed via an attribute.

To verify that it works, i also added chefspec tests. But only for a debian system for now. It must works for other system too.
can this be changed to the following, style wise?

    include_recipe 'varnish::repo' if node['varnish']['use_default_repo']