Deb: Add the release codename to the release
Deb: Add the release codename to the release

This is similar to `%{dist}` in RHEL/CentOS. However, we need to parse
this from /etc/os-release.

Tested on:
 * Debian Jessie
 * Debian Stretch
 * Ubuntu Trusty
 * Ubuntu Xenial
 * Ubuntu Zesty
 * Ubuntu Artful

PowerDNS/pdns-builder,PowerDNS/pdns-builder