admin: Don't erase challenge.json if already exists

admin/main.go

@@ -144,6 +144,7 @@ func main() {
 		log.Println("Using", sync.GlobalImporter.Kind())
 
 		// Update distributed challenge.json
+		if _, err := os.Stat(path.Join(settings.SettingsDir, settings.ChallengeFile)); os.IsNotExist(err) {
 			challengeinfo, err := sync.GetFileContent(sync.GlobalImporter, settings.ChallengeFile)
 			if err == nil {
 				if fd, err := os.Create(path.Join(settings.SettingsDir, settings.ChallengeFile)); err != nil {
@@ -157,6 +158,7 @@ func main() {
 				}
 			}
 		}
+	}
 
 	// Sanitize options
 	var err error