Partial fix for #4565. Works only for Debian 7 and later

Determines the correct `ansible_distribution_release` for Debian 7 and later. It works for Debian stable/testing/sid.

I didn't find a clean solution for Debian versions <7 as `/etc/os-release` didn't exist there yet.

Maybe "hardcoding" a few codenames and comparing them to the first number in `ansible_distribution_version` is acceptable?