Change log format to JSON and log destination to stdout (#48)

* log: change log format to JSON payload for better log in Stackdriver

change the log format in Go written service from text payload to
JSON payload using 3rd party logging library (logrus).

https://cloud.google.com/logging/docs/structured-logging
33a1e118e1/json_formatter.go (L40-L49)

Effected services are frontend, productcatalogservice, checkoutservice,
and shippinservice.

Also change target container registry and locust scenario for testing.

* revert kubernetes manifests to point to the original container registry URLs

* revert skaffold.yaml to point to the original registry

* loadgenerator: revert locust settings

src/frontend/main.go

@@ -83,7 +83,15 @@ func main() {
 	ctx := context.Background()
 	log := logrus.New()
 	log.Level = logrus.DebugLevel
-	log.Formatter = &logrus.TextFormatter{}
+	log.Formatter = &logrus.JSONFormatter{
+		FieldMap: logrus.FieldMap{
+			logrus.FieldKeyTime:  "timestamp",
+			logrus.FieldKeyLevel: "severity",
+			logrus.FieldKeyMsg:   "message",
+		},
+		TimestampFormat: time.RFC3339Nano,
+	}
+	log.Out = os.Stdout
 
 	go initProfiling(log, "frontend", "1.0.0")
 	go initTracing(log)