**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
if cls._context_managed_dag:
            cls._previous_context_managed_dags.append(cls._context_managed_dag)
        cls._context_managed_dag = dag
    @classmethod
    def pop_context_managed_dag(cls) -> Optional[DAG]:
        old_dag = cls._context_managed_dag
        if cls._previous_context_managed_dags:
            cls._context_managed_dag = cls._previous_context_managed_dags.pop()
        else:
            cls._context_managed_dag = None
        return old_dag
    @classmethod
    def get_current_dag(cls) -> Optional[DAG]:
        return cls._context_managed_dag