[SPARK-2514] [mllib] Random RDD generator

Utilities for generating random RDDs.

RandomRDD and RandomVectorRDD are created instead of using `sc.parallelize(range:Range)` because `Range` objects in Scala can only have `size <= Int.MaxValue`. 

The object `RandomRDDGenerators` can be transformed into a generator class to reduce the number of auxiliary methods for optional arguments. 
We need to be consistent on the argument name. `distribution` is used here but `rng` is used in `randomVectorRDD`. `generator` sounds better to me than `distribution` because `DistributionGenerator` is a generator but not a distribution and in the context we don't need to use `distributionGenerator`.