[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. 
As with most random objects, the DistributionGenerator should be created with a default seed (so using it before calling `setSeed` is legal). I like how in Colt it's called `reseed` instead, but `setSeed` is also widely adopted.