:return: all knows OS instance

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