add nixops deployment

nixops/single-server.nix

@@ -0,0 +1,11 @@
+{
+  network.description = "mail server";
+
+  mailserver =
+    { config, pkgs, ... }:
+    {
+        imports = [
+            ./../mail-config.nix
+        ];
+    };
+}

nixops/vbox.nix

@@ -0,0 +1,9 @@
+{
+  mailserver =
+    { config, pkgs, ... }:
+    { deployment.targetEnv = "virtualbox";
+      deployment.virtualbox.memorySize = 1024; # megabytes
+      deployment.virtualbox.vcpu = 2; # number of cpus
+      deployment.virtualbox.headless = true;
+    };
+}