[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.
For these tests to run automatically, you also need to add this file into the `python/run-tests` script. Otherwise it won't automatically discover it, e.g. in Jenkins.