Refactor common modules.

common/nomad-server.nix

@@ -0,0 +1,9 @@
+{ ... }:
+{
+  # Enable server mode for both Consul and Nomad
+  # Used by: c1, c2, c3 (quorum members)
+  clusterRole = {
+    consulServer = true;
+    nomadServer = true;
+  };
+}