PySpark Bindings for K8S - make Python 3 the default

Initial PySpark-k8s bindings have just been resolved (SPARK-23984), but the default Python version there is 2. While you can override this by setting it to 3, I think we should have sensible defaults.

Python 3 has been around for ten years and is the clear successor, Python 2 has only 18 months left in terms of support. There isn't a good reason to suggest Python 2 should be used, not in 2018 and not when both versions are supported.

The relevant commit is here, the version is also in the documentation.