Motivation for this change

I'm running zookeeper in a container on Nixos and noticed that the zookeeper package was not installed in there. I therefore adapted the zookeeper service to me more like the kafka service:

• make the actual package that is used configurable
• add the used package to the systemPackages

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.