[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.
You could actually leave out the `toJavaRDD()`s here and use the implicits defined in `JavaRDD`; we do this in a lot of the other Java APIs, including JavaRDD: https://github.com/dorx/spark/blob/pythonRDD/core/src/main/scala/org/apache/spark/api/java/JavaRDD.scala#L88