fix(kafka): update security protocol to use default configuration

Author: incubator4

plugin-server/src/kafka/consumer.ts

@@ -131,7 +131,7 @@ export class KafkaConsumer {
 
         this.consumerConfig = {
             'client.id': hostname(),
-            'security.protocol': 'plaintext',
+            'security.protocol': defaultConfig.KAFKA_SECURITY_PROTOCOL,
             'metadata.broker.list': 'kafka:9092', // Overridden with KAFKA_CONSUMER_METADATA_BROKER_LIST
             log_level: 4, // WARN as the default
             'group.id': this.config.groupId,