makeImageFromDebDist: Add extraDebs arguments

This allows adding packages that are not part of the distribution, e.g.g

  extraDebs = [
    (pkgs.fetchurl {
      name = "openjdk.deb";
      url = http://ppa.launchpad.net/openjdk-r/ppa/ubuntu/pool/main/o/openjdk-8/openjdk-8-jdk-headless_8u111-b14-3~14.04.1_amd64.deb;
      sha256 = "1n5ibpkx9pjmc4nr052rls1yqbq7ckav2rabixjhd4yxbyhjl0ap";
    })
  ];