default task number misleading in several places

  private[streaming] def defaultPartitioner(numPartitions: Int = self.ssc.sc.defaultParallelism){
    new HashPartitioner(numPartitions)
  }

it represents that the default task number in Spark Streaming relies on the variable defaultParallelism in SparkContext, which is decided by the config property spark.default.parallelism

the property "spark.default.parallelism" refers to https://github.com/apache/spark/pull/389

Author: Chen Chao <crazyjvm@gmail.com>

Closes #766 from CrazyJvm/patch-7 and squashes the following commits:

0b7efba [Chen Chao] Update streaming-programming-guide.md
cc5b66c [Chen Chao] default task number misleading in several places

(cherry picked from commit 2f639957f0bf70dddf1e698aa9e26007fb58bc67)
Signed-off-by: Reynold Xin <rxin@apache.org>