**Apache Airflow version**:
2.0.1

**Kubernetes version**:
Server Version: version.Info{Major:"1", Minor:"18+", GitVersion:"v1.18.9-eks-d1db3c", GitCommit:"d1db3c46e55f95d6a7d3e5578689371318f95ff9", GitTreeState:"clean", BuildDate:"2020-10-20T22:18:07Z", GoVersion:"go1.13.15", Compiler:"gc", Platform:"linux/amd64"}

**What happened**:
Clearing a failed subdag task with Downstream+Recursive does not automatically set the state of the parent dag to 'running' so that the downstream parent tasks can execute.

The work around is to manually set the state of the parent dag to running after clearing the subdag task

**What you expected to happen**:
With airflow version 1.10.4 the parent dag was automatically set to 'running' for this same scenario

**How to reproduce it**:
- Clear a failed subdag task selecting option for Downstream+Recursive
- See that all down stream tasks in the subdag as well as the parent dag have been cleared
- See that the parent dag is left in 'failed' state. 

airflow/models/dag.py
t._upstream_task_ids = t.upstream_task_ids.intersection(dag.task_dict.keys())
            t._downstream_task_ids = t.downstream_task_ids.intersection(dag.task_dict.keys())
        if len(dag.tasks) < len(self.tasks):
            dag.partial = True
        return dag
    def has_task(self, task_id: str):
        return task_id in (t.task_id for t in self.tasks)
    def get_task(self, task_id: str, include_subdags: bool = False) -> BaseOperator:
        if task_id in self.task_dict:
            return self.task_dict[task_id]
        if include_subdags:
            for dag in self.subdags:
                if task_id in dag.task_dict:
                    return dag.task_dict[task_id]
        raise TaskNotFound(f"Task {task_id} not found")
    def pickle_info(self):
        d = {}
        d['is_picklable'] = True
        try:
            dttm = timezone.utcnow()
            pickled = pickle.dumps(self)
            d['pickle_len'] = len(pickled)
            d['pickling_duration'] = str(timezone.utcnow() - dttm)
        except Exception as e:
            self.log.debug(e)
            d['is_picklable'] = False
            d['stacktrace'] = traceback.format_exc()
        return d
    @provide_session
    def pickle(self, session=None) -> DagPickle: