use python_version = PY2 for par_binary and py_binary targets

[nlopezgi]

Addresses bazelbuild/rules_k8s#305
Sending PR to kick off presubmits and also check if rules_k8s will be fixed by changing pin to this commit + adding some more python_version = PY2 there.

[k8s-ci-robot]

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[nlopezgi]

This PR still needs a bit more documentation and organizing stuff. Please hold off reviewing.

[nlopezgi]

I also need to change how the default_py_runtime is generated, right now it has a hardcoded path that will not work with travisCI setup. Will need to create a repo rule that uses which to find location of py2 and py3 binaries.

[nlopezgi]

closing in favor of #826