[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.
If we can use absolute import to get `random.randint`, can we use it to generate the default seed? It is not necessary to have the default seed evenly distributed across the long range. Some thing like `long(randint(-2e32, 2e32))` should be sufficiently good and we don't need to worry about having more than 64 bits.