kubernetes vm test: make dig and netcat available

Dig could not be found in the test cases. Adding it as a global package
fixes this.

Changed files
+1
nixos
+1
nixos/tests/kubernetes.nix
···
virtualisation.diskSize = 2048;
programs.bash.enableCompletion = true;
+
environment.systemPackages = with pkgs; [ netcat bind ];
services.kubernetes.roles = ["master" "node"];
virtualisation.docker.extraOptions = "--iptables=false --ip-masq=false -b cbr0";