[SPARK-4837] NettyBlockTransferService should use spark.blockManager.port config

This is used in NioBlockTransferService here:
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/network/nio/NioBlockTransferService.scala#L66

Author: Aaron Davidson <aaron@databricks.com>

Closes #3688 from aarondav/SPARK-4837 and squashes the following commits:

ebd2007 [Aaron Davidson] [SPARK-4837] NettyBlockTransferService should use spark.blockManager.port config