update spark.default.parallelism

actually, the value 8 is only valid in mesos fine-grained mode :
<code>
  override def defaultParallelism() = sc.conf.getInt("spark.default.parallelism", 8)
</code>

while in coarse-grained model including mesos coares-grained, the value of the property depending on core numbers!
<code> 
override def defaultParallelism(): Int = {
   conf.getInt("spark.default.parallelism", math.max(totalCoreCount.get(), 2))
  }
</code>