docs: def get_attribute(self, attribute):
          """
          :param attribute: requested attributes.
          :return: attribute value.
          :raise TgnError: if invalid attribute.
          """
          value = self.api.getAttribute(self.obj_ref(), attribute)
          # IXN returns '::ixNet::OK' for invalid attributes. We want error.
          if value == '::ixNet::OK':
              raise TgnError(self.ref + ' does not have attribute ' + attribute)
          return str(value)