[SPARK-2801][MLlib]: DistributionGenerator renamed to RandomDataGenerator. RandomRDD is now of generic type

The RandomRDDGenerators used to only output RDD[Double]. 
Now RandomRDDGenerators.randomRDD can be used to generate a random RDD[T] via a class that extends RandomDataGenerator, by supplying a type T and overriding the nextValue() function as they wish.