[Core] Adding a parallelismRatio to control the partitions num of shuffledRDD

Adding parallelismRatio to control the partitions num of shuffledRDD, the rule is:
  
     Math.max(1, parallelismRatio * number of partitions of the largest upstream RDD)
  
The ratio is 1.0 by default to make it compatible with the old version. When we have a good experience on it, we can change this.