[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

core/src/main/scala/org/apache/spark/network/netty/NettyBlockTransferService.scala

@@ -60,7 +60,7 @@ class NettyBlockTransferService(conf: SparkConf, securityManager: SecurityManage
     }
     transportContext = new TransportContext(transportConf, rpcHandler)
     clientFactory = transportContext.createClientFactory(bootstrap.toList)
-    server = transportContext.createServer()
+    server = transportContext.createServer(conf.getInt("spark.blockManager.port", 0))
     appId = conf.getAppId
     logInfo("Server created on " + server.getPort)
   }