[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. 
If `numPartiitons` > `size`, there would be empty partitions. We should allow this case, because it happens when a user uses the default number of partitions, which may be greater than `size`.