[SPARK-2724] Python version of RandomRDDGenerators

RandomRDDGenerators but without support for randomRDD and randomVectorRDD, which take in arbitrary DistributionGenerator.

`randomRDD.py` is named to avoid collision with the built-in Python `random` package.
Change this to a descriptive comment, e.g. "random data generation routines", rather than saying it's a wrapper around another package. We want the Python docs to stand by themselves for people without Scala / Java experience.