Make __init__.py generation optional in protoc-all

[mdkess]

As of Python 3.6, __init__.py files are not required. This task is to make it optional to generate those files. This will change the behavior around

docker-protoc/all/entrypoint.sh

Line 133 in e2dcfde

if [[ $GEN_LANG == "python" ]]; then

This should be done by passing a command line flag to entrypoint.sh. The default should be to generate those files, to maintain backward compatibility with existing users.