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/azure-image.nix

@@ -26,7 +26,7 @@ in
               ${pkgs.vmTools.qemu}/bin/qemu-img convert -f raw -O vpc $diskImage $out/disk.vhd
               rm $diskImage
             '';
-          diskImageBase = "nixos-image-${config.system.nixosVersion}-${pkgs.stdenv.system}.raw";
+          diskImageBase = "nixos-image-${config.system.nixosLabel}-${pkgs.stdenv.system}.raw";
           buildInputs = [ pkgs.utillinux pkgs.perl ];
           exportReferencesGraph =
             [ "closure" config.system.build.toplevel ];