[SPARK-21168] KafkaRDD should always set kafka clientId. apache#19887

https://issues.apache.org/jira/browse/SPARK-21168
There are no a number of other places that a client ID should be set,and I think we should use consumer.clientId in the clientId method,because the fetch request will be used by the same consumer behind.

external/kafka-0-8/src/main/scala/org/apache/spark/streaming/kafka/KafkaRDD.scala

@@ -191,6 +191,7 @@ class KafkaRDD[
 
     private def fetchBatch: Iterator[MessageAndOffset] = {
       val req = new FetchRequestBuilder()
+        .clientId(consumer.clientId)
         .addFetch(part.topic, part.partition, requestOffset, kc.config.fetchMessageMaxBytes)
         .build()
       val resp = consumer.fetch(req)