Use http.debian.net as a default mirror

http.debian.net is a redirector that uses the geo and network location
of the user and the mirrors.
Test that sources.list is not managed on non-debians
Don't rely on `lsb_release` being installed

If "lsb-release" package is not installed and `node['lsb']['codename']`
thus nil, try to get the codename from a list.

Use `node['debian']['codename']` attribute to store the codename.
Leave it undocumented as it is not meant to be managed by the user.

Fixes #6.