Open files to be generated with truncation.

pkg/varnish/vcl/template.go

@@ -71,7 +71,8 @@ func replIllegal(ill []byte) []byte {
 // WriteTemplate generates a VCL file at path to implement spec, using
 // the template.
 func (spec Spec) WriteTemplate(tmpl *template.Template, path string) error {
-	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE, 0644)
+	file, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_TRUNC,
+		0644)
 	if err != nil {
 		return err
 	}