docs: def get_os_instances():
          """
          :return: all knows OS instance
          """
          LOGGER.debug("OSInstanceService.get_os_instances")
          args = {'http_operation': 'GET', 'operation_path': ''}
          response = OSInstanceService.requester.call(args)
          ret = None
          if response.rc == 0:
              ret = []
              for osInstance in response.response_content['osInstances']:
                  ret.append(OSInstance.json_2_os_instance(osInstance))
          elif response.rc != 404:
              err_msg = 'OSInstanceService.get_os_instances - Problem while getting os instances. ' \
                        '. Reason: ' + str(response.response_content) + '-' + str(response.error_message) + \
                        " (" + str(response.rc) + ")"
              LOGGER.warning(err_msg)
          return ret