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**Description**

Related to the discussion here: https://github.com/apache/airflow/pull/8553#discussion_r416136954

Allow user to define expected terminal state. e.g. `JOB_STATE_CANCELLED` could be not treated as failure but expected terminal state.


<!-- A short description of your feature -->


if key in environment:
                self.log.warning(
                    "%r parameter in 'variables' will override the same one passed in 'environment'!",
                    key,
                )
            return key, val
        environment.update(_check_one(key, val) for key, val in variables.items() if key in environment_keys)
        return environment
    @GoogleBaseHook.fallback_to_default_project_id
    def start_flex_template(