def get_repo_slug(repo_owner=None, repo_name=None):
    """
    Return repo slug.

    This function concatenates repo_owner and repo_name,
    fe. buildtimetrend/service

    Parameters :
    - repo_owner : name of the Github repo owner, fe. `buildtimetrend`
    - repo_name : name of the Github repo, fe. `service`
    """
    if repo_owner is not None and repo_name is not None:
        return '%s/%s' % (str(repo_owner), str(repo_name))
    else:
        return None
