[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. 
I'm not sure this is a great idea unfortunately, see http://stackoverflow.com/questions/426350/seeding-java-util-random-with-consecutive-numbers. I think it would be safer to use a RNG to generate the seeds. In tests you can just make sure that "most" of them are distinct instead, or you can test for the specific sequence expected by our generator.