nixos: introduce system.nixosLabel option and use it where appropriate

Setting nixosVersion to something custom is useful for meaningful GRUB
menus and /nix/store paths, but actuallly changing it rebulids the
whole system path (because of `nixos-version` script and manual
pages). Also, changing it is not a particularly good idea because you
can then be differentitated from other NixOS users by a lot of
programs that read /etc/os-release.

This patch introduces an alternative option that does all you want
from nixosVersion, but rebuilds only the very top system level and
/etc while using your label in the names of system /nix/store paths,
GRUB and other boot loaders' menus, getty greetings and so on.

nixos/modules/virtualisation/virtualbox-image.nix

@@ -44,8 +44,8 @@ in {
 
     system.build.virtualBoxOVA = pkgs.runCommand "virtualbox-ova"
       { buildInputs = [ pkgs.linuxPackages.virtualbox ];
-        vmName = "NixOS ${config.system.nixosVersion} (${pkgs.stdenv.system})";
-        fileName = "nixos-image-${config.system.nixosVersion}-${pkgs.stdenv.system}.ova";
+        vmName = "NixOS ${config.system.nixosLabel} (${pkgs.stdenv.system})";
+        fileName = "nixos-image-${config.system.nixosLabel}-${pkgs.stdenv.system}.ova";
       }
       ''
         echo "creating VirtualBox VM..."