docs: def app_token(vault_client, app_id, user_id):
      """Returns a vault token based on the app and user id."""
      resp = vault_client.auth_app_id(app_id, user_id)
      if 'auth' in resp and 'client_token' in resp['auth']:
          return resp['auth']['client_token']
      else:
          raise aomi.exceptions.AomiCredentials('invalid apptoken')