retrieve a list of all devices that exist in nsot

    :param api_url: str
    :param email: str
    :param secret_key: str
    :return: dict

def _all_nsot_devices(api_url, email, secret_key):
    '''
    retrieve a list of all devices that exist in nsot

    :param api_url: str
    :param email: str
    :param secret_key: str
    :return: dict
    '''
    token = _get_token(api_url, email, secret_key)
    all_devices = {}
    if token:
        headers = {'Authorization': 'AuthToken {}:{}'.format(email, token)}
        all_devices = _query_nsot(api_url, headers)

    return all_devices