When environment_type=PROCESS, environment_config specifies the command to run the worker processes. Right now, it defaults to None and errors if not set (`TypeError: expected string or buffer`).
It might not be feasible to offer a one-size-fits-all executable for providing as environment_config, but we could at least:
a) make it easier to build one (right now I only see the executable being built in a test script that depends on docker: https://github.com/apache/beam/blob/cbf8a900819c52940a0edd90f59bf6aec55c817a/sdks/python/test-suites/portable/py2/build.gradle#L146-L165)
b) document the process
c) link to the documentation when no environment_config is provided