* and stop setting default values in python operator code, so defaults can be set at the server side.

This issue is just for the PR and questions from maintainers. 
setup.py
return f'.release:{version_}+{sha}'
    return 'no_git_version'
def write_version(filename: str = os.path.join(*[my_dir, "airflow", "git_version"])) -> None:
    """
    Write the Semver version + git hash to file, e.g. ".dev0+2f635dc265e78db6708f59f68e8009abb92c1e65".
    :param str filename: Destination file to write
    """
    text = f"{git_version(version)}"
    with open(filename, 'w') as file:
        file.write(text)
amazon = [
    'boto3>=1.15.0,<1.18.0',
    'watchtower~=1.0.6',
]
apache_beam = [
    'apache-beam>=2.20.0',
]
asana = ['asana>=0.10', 'cached-property>=1.5.2']
async_packages = [
    'eventlet>= 0.9.7',
    'gevent>=0.13',
    'greenlet>=0.4.9',
]
atlas = [
    'atlasclient>=0.1.2',
]
azure = [
    'azure-batch>=8.0.0',
    'azure-cosmos>=3.0.1,<4',
    'azure-datalake-store>=0.0.45',