Support a "defaults" attribute that sets default values for all machines

The "defaults" attribute in every network expressions is stacked on
top of every machine configuration.  Ideally the values defined in
"defaults" would have a lower priority, but I don't think it's
possible yet to set a default priority for all values in a NixOS
module.

examples/apache-ec2.nix

@@ -1,17 +1,12 @@
-let
-
-  config =
+{
+  defaults =
+    { config, pkgs, ... }:
     { deployment.targetEnv = "ec2";
-      deployment.ec2.region = "us-east-1";
-      deployment.ec2.instanceType = "m1.large";
+      deployment.ec2.region = pkgs.lib.mkDefault "eu-west-1";
+      deployment.ec2.instanceType = "m1.small";
       deployment.ec2.keyPair = "eelco";
       deployment.ec2.securityGroups = [ "eelco-test" ];
     };
-
-in
-
-{
-  proxy = config;
-  backend1 = config;
-  backend2 = config;
+
+  backend2.deployment.ec2.region = "us-west-1";
 }

nix/eval-machine-info.nix

@@ -11,28 +11,31 @@ rec {
   networks = map (networkExpr: import networkExpr) networkExprs;
 
   network = zipAttrs networks;
+
+  defaults = network.defaults or [];
 
   nodes =
-    listToAttrs (map (configurationName:
+    listToAttrs (map (machineName:
       let
-        modules = getAttr configurationName network;
+        modules = getAttr machineName network;
       in
-      { name = configurationName;
+      { name = machineName;
         value = import <nixos/lib/eval-config.nix> {
           modules =
             modules ++
+            defaults ++
             [ # Provide a default hostname and deployment target equal
               # to the attribute name of the machine in the model.
               { key = "set-default-hostname";
-                networking.hostName = mkOverride 900 configurationName;
-                deployment.targetHost = mkOverride 900 configurationName;
+                networking.hostName = mkOverride 900 machineName;
+                deployment.targetHost = mkOverride 900 machineName;
                 environment.checkConfigurationOptions = false; # should only do this in phase 1
               }
             ];
           extraArgs = { inherit nodes; };
         };
       }
-    ) (attrNames (removeAttrs network [ "network" ])));
+    ) (attrNames (removeAttrs network [ "network" "defaults" ])));
 
   # Phase 1: evaluate only the deployment attributes.
   info = {