Added struct to be passed to the Process function to render the vagrant file...

Added struct to be passed to the Process function to render the vagrant file path.  Resolves Issue #1125.

post-processor/vagrant/post-processor.go

@@ -125,9 +125,17 @@ func (p *PostProcessor) PostProcess(ui packer.Ui, artifact packer.Artifact) (pac
 	// Write our Vagrantfile
 	var customVagrantfile string
 	if config.VagrantfileTemplate != "" {
+		vagrantfilePath, err := config.tpl.Process(config.VagrantfileTemplate, &vagrantfileTemplate{
+			ProviderVagrantfile: vagrantfile,
+			CustomVagrantfile: "",
+		})
+		if err != nil {
+			return nil, false, err
+		}
+
 		ui.Message(fmt.Sprintf(
-			"Using custom Vagrantfile: %s", config.VagrantfileTemplate))
-		customBytes, err := ioutil.ReadFile(config.VagrantfileTemplate)
+			"Using custom Vagrantfile: %s", vagrantfilePath))
+		customBytes, err := ioutil.ReadFile(vagrantfilePath)
 		if err != nil {
 			return nil, false, err
 		}