**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
"""
        Returns a list of dag run execution dates currently running
        :return: List of execution dates
        """
        runs = DagRun.find(dag_id=self.dag_id, state=State.RUNNING)
        active_dates = []
        for run in runs:
            active_dates.append(run.execution_date)
        return active_dates
    @provide_session
    def get_num_active_runs(self, external_trigger=None, session=None):
        """
        Returns the number of active "running" dag runs
        :param external_trigger: True for externally triggered active dag runs
        :type external_trigger: bool
        :param session:
        :return: number greater than 0 for active dag runs
        """
        
        query = (
            session.query(func.count())
            .filter(DagRun.dag_id == self.dag_id)
            .filter(DagRun.state == State.RUNNING)
        )
        if external_trigger is not None:
            query = query.filter(DagRun.external_trigger == external_trigger)
        return query.scalar()
    @provide_session
    def get_dagrun(self, execution_date, session=None):