Minor cleanup in samples and tools

- Cleanup in schema-registry samples (final variable).
- Add instructions in tools/kafka/docker-compose to run
  a quick test using Kafka console scripts.

samples/spring-cloud-stream-schema-registry-integration/schema-registry-producer1-kafka/src/main/java/sample/producer1/Producer1ApplicationKafka.java

@@ -35,7 +35,7 @@ import org.springframework.web.bind.annotation.RestController;
 @RestController
 public class Producer1ApplicationKafka {
 
-	private Random random = new Random();
+	private final Random random = new Random();
 
 	@Autowired
 	private StreamBridge streamBridge;

samples/spring-cloud-stream-schema-registry-integration/schema-registry-producer2-kafka/src/main/java/sample/producer2/Producer2ApplicationKafka.java

@@ -41,7 +41,7 @@ import com.example.Sensor;
 @RestController
 public class Producer2ApplicationKafka {
 
-	private Random random = new Random();
+	private final Random random = new Random();
 
 	@Autowired
 	private StreamBridge streamBridge;