[SPARK-20600][SS] KafkaRelation should be pretty printed in web UI

## What changes were proposed in this pull request?

User-friendly name of `KafkaRelation` in web UI (under Details for Query).

### Before

<img width="516" alt="spark-20600-before" src="https://cloud.githubusercontent.com/assets/62313/25841955/74479ac6-34a2-11e7-87fb-d9f62a1356a7.png">

### After

<img width="439" alt="spark-20600-after" src="https://cloud.githubusercontent.com/assets/62313/25841829/f5335630-34a1-11e7-85a4-afe9b66d73c8.png">

## How was this patch tested?

Local build

```
./bin/spark-shell --jars ~/.m2/repository/org/apache/spark/spark-sql-kafka-0-10_2.11/2.3.0-SNAPSHOT/spark-sql-kafka-0-10_2.11-2.3.0-SNAPSHOT.jar --packages org.apache.kafka:kafka-clients:0.10.0.1
```

Author: Jacek Laskowski <jacek@japila.pl>

Closes #17917 from jaceklaskowski/SPARK-20600-KafkaRelation-webUI.

external/kafka-0-10-sql/src/main/scala/org/apache/spark/sql/kafka010/KafkaRelation.scala

@@ -143,4 +143,7 @@ private[kafka010] class KafkaRelation(
         validateTopicPartitions(partitions, partitionOffsets)
     }
   }
+
+  override def toString: String =
+    s"KafkaRelation(strategy=$strategy, start=$startingOffsets, end=$endingOffsets)"
 }