Add option to allow box Vagrantfile to be dynamically generated during build

post-processor/vagrant/post-processor.go

@@ -47,6 +47,7 @@ type Config struct {
 	OutputPath                   string   `mapstructure:"output"`
 	Override                     map[string]interface{}
 	VagrantfileTemplate          string `mapstructure:"vagrantfile_template"`
+	VagrantfileTemplateGenerated bool   `mapstructure:"vagrantfile_template_generated"`
 
 	ctx interpolate.Context
 }
@@ -217,7 +218,7 @@ func (p *PostProcessor) configureSingle(c *Config, raws ...interface{}) error {
 	}
 
 	var errs *packer.MultiError
-	if c.VagrantfileTemplate != "" {
+	if c.VagrantfileTemplate != "" && c.VagrantfileTemplateGenerated == false {
 		_, err := os.Stat(c.VagrantfileTemplate)
 		if err != nil {
 			errs = packer.MultiErrorAppend(errs, fmt.Errorf(