cluster: rename module-* into modules/*

Change-Id: I65e06f3e9cec2ba0071259eb755eddbbd1025b97
diff --git a/cluster/nix/provision.nix b/cluster/nix/provision.nix
index dbe697b..70240f6 100644
--- a/cluster/nix/provision.nix
+++ b/cluster/nix/provision.nix
@@ -7,8 +7,8 @@
     value = super.nixos ({ config, pkgs, ... }: {
       networking.hostName = machine.name;
       imports = [
-        ./module-base.nix
-        ./module-kubernetes.nix
+        ./modules/base.nix
+        ./modules/kubernetes.nix
       ];
     });
   }) machines);