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.
Yes, i often put a low value here, to verify easily some memory rules calculation. But as there's none of that in this cookbook, i can remove it if needed.