Support Instance SA authentication

builder/yandex/config.go

@@ -248,11 +248,6 @@ func (c *Config) Prepare(raws ...interface{}) ([]string, error) {
 		c.FolderID = os.Getenv("YC_FOLDER_ID")
 	}
 
-	if c.Token == "" && c.ServiceAccountKeyFile == "" {
-		errs = packer.MultiErrorAppend(
-			errs, errors.New("a token or service account key file must be specified"))
-	}
-
 	if c.Token != "" && c.ServiceAccountKeyFile != "" {
 		errs = packer.MultiErrorAppend(
 			errs, errors.New("one of token or service account key file must be specified, not both"))

builder/yandex/driver_yc.go

@@ -43,10 +43,16 @@ func NewDriverYC(ui packer.Ui, config *Config) (Driver, error) {
 	}
 
 	switch {
+	case config.Token == "" && config.ServiceAccountKeyFile == "":
+		log.Printf("[INFO] Use Instance Service Account for authentication")
+		sdkConfig.Credentials = ycsdk.InstanceServiceAccount()
+
 	case config.Token != "":
+		log.Printf("[INFO] Use OAuth token for authentication")
 		sdkConfig.Credentials = ycsdk.OAuthToken(config.Token)
 
 	case config.ServiceAccountKeyFile != "":
+		log.Printf("[INFO] Use Service Account key file %q for authentication", config.ServiceAccountKeyFile)
 		key, err := iamkey.ReadFromJSONFile(config.ServiceAccountKeyFile)
 		if err != nil {
 			return nil, err