Use default subnet when creating IG

We had this logic before, but I accidentally bypassed it in #2124.

cmd/kops/create_ig.go

@@ -150,19 +150,15 @@ func RunCreateInstanceGroup(f *util.Factory, cmd *cobra.Command, args []string,
 	}
 	ig.Spec.Role = role
 
-	if len(options.Subnets) == 0 {
-		return fmt.Errorf("cannot create instance group without subnets; specify --subnet flag(s)")
-	}
 	ig.Spec.Subnets = options.Subnets
 
 	ig, err = cloudup.PopulateInstanceGroupSpec(cluster, ig, channel)
-
-	ig.AddInstanceGroupNodeLabel()
-
 	if err != nil {
 		return err
 	}
 
+	ig.AddInstanceGroupNodeLabel()
+
 	if options.DryRun {
 
 		if options.Output == "" {