* 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 a version to identify the state of the underlying git repo. The version will
    indicate whether the head of the current git-backed working directory is tied to a
    release tag or not : it will indicate the former with a 'release:{version}' prefix
    and the latter with a '.dev0' suffix. Following the prefix will be a sha of the current
    branch head. Finally, a "dirty" suffix is appended to indicate that uncommitted
    changes are present.
    :param str version_: Semver version
    :return: Found Airflow version in Git repo
    :rtype: str
    """
    try:
        import git
        try:
            repo = git.Repo(os.path.join(*[my_dir, '.git']))
        except git.NoSuchPathError:
            logger.warning('.git directory not found: Cannot compute the git version')
            return ''
        except git.InvalidGitRepositoryError:
            logger.warning('Invalid .git directory not found: Cannot compute the git version')
            return ''
    except ImportError:
        logger.warning('gitpython not found: Cannot compute the git version.')
        return ''
    if repo:
        sha = repo.head.commit.hexsha
        if repo.is_dirty():
            return f'.dev0+{sha}.dirty'