Fetches a list of all DSCP marking rules for the given policy.

def list_dscp_marking_rules(self, policy_id,
                                retrieve_all=True, **_params):
        """Fetches a list of all DSCP marking rules for the given policy."""
        return self.list('dscp_marking_rules',
                         self.qos_dscp_marking_rules_path % policy_id,
                         retrieve_all, **_params)