Returns a macaroon from a legacy wait endpoint.

def _wait_for_macaroon(wait_url):
    ''' Returns a macaroon from a legacy wait endpoint.
    '''
    headers = {
        BAKERY_PROTOCOL_HEADER: str(bakery.LATEST_VERSION)
    }
    resp = requests.get(url=wait_url, headers=headers)
    if resp.status_code != 200:
        raise InteractionError('cannot get {}'.format(wait_url))

    return bakery.Macaroon.from_dict(resp.json().get('Macaroon'))