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.
I understand, but as it's just unit testing, without trying to execute any real install... It's easyer for me to calculate only the rule for a small amount of RAM :D