[SPARK-11063] [STREAMING] Change preferredLocations of Receiver's RDD to hosts rather than hostports

The format of RDD's preferredLocations must be hostname but the format of Streaming Receiver's scheduling executors is hostport. So it doesn't work.

This PR converts `schedulerExecutors` to `hosts` before creating Receiver's RDD.

Author: zsxwing <zsxwing@gmail.com>

Closes #9075 from zsxwing/SPARK-11063.