Get a temporary auth token from LTD Keeper.

def get_keeper_token(base_url, username, password):
    """Get a temporary auth token from LTD Keeper."""
    token_endpoint = base_url + '/token'
    r = requests.get(token_endpoint, auth=(username, password))
    if r.status_code != 200:
        raise RuntimeError('Could not authenticate to {0}: error {1:d}\n{2}'.
                           format(base_url, r.status_code, r.json()))
    return r.json()['token']