[SPARK-2492][Streaming] kafkaReceiver minor changes to align with Kafka 0.8

Update the KafkaReceiver's behavior when auto.offset.reset is set. 

In Kafka 0.8, `auto.offset.reset` is a hint for out-range offset to seek to the beginning or end of the partition. While in the previous code `auto.offset.reset` is a enforcement to seek to the beginning or end immediately, this is different from Kafka 0.8 defined behavior. 

Also deleting extesting ZK metadata in Receiver when multiple consumers are launched will introduce issue as mentioned in [SPARK-2383](https://issues.apache.org/jira/browse/SPARK-2383).

So Here we change to offer user to API to explicitly reset offset before create Kafka stream, while in the meantime keep the same behavior as Kafka 0.8 for parameter `auto.offset.reset`.

@tdas, would you please review this PR? Thanks a lot.
I dont think it is good idea to expose have this utility function in our API that is very specific to a particular version of Kafka. If this functionality is required for some well-known version-specific Kafka issue, then I am guessing there will be known workarounds in the Kafka-world and we dont need to provide custom functionality for that. So I dont think we should add this.