:return: all knows OS types

def get_ostypes():
        """
        :return: all knows OS types
        """
        LOGGER.debug("OSTypeService.get_ostypes")
        args = {'http_operation': 'GET', 'operation_path': ''}
        response = OSTypeService.requester.call(args)
        ret = None
        if response.rc == 0:
            ret = []
            for os_type in response.response_content['osTypes']:
                ret.append(OSType.json_2_ostype(os_type))
        elif response.rc != 404:
            err_msg = 'OSTypeService.get_ostypes - Problem while getting OS Types.' \
                      '. Reason: ' + str(response.response_content) + '-' + str(response.error_message) + \
                      " (" + str(response.rc) + ")"
            LOGGER.warning(err_msg)

        return ret