Move common nix code one level up.

common/glusterfs.nix

@@ -0,0 +1,13 @@
+{ pkgs, config, lib, ... }:
+{
+  services.glusterfs = {
+    enable = true;
+  };
+
+  environment.persistence."/persist".directories = [
+    "/var/lib/glusterd"
+  ];
+
+  # TODO: each volume needs its own port starting at 49152
+  networking.firewall.allowedTCPPorts = [ 24007 24008 24009 49152 49153 49154 49155 ];
+}