**Apache Airflow version**: 2.0.0a2

**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.15.12

**Environment**:
- **OS** (e.g. from /etc/os-release): CentOS 7


**What happened**:

When using KubernetesExecutor, a single SIGTERM doesn't result in the scheduler actually exiting cleanly. However, it will exit after receiving a second SIGTERM.

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in `pod_mutation_hook`.

In practice, this means the scheduler doesn't get restarted by K8s after encountering a exception. Or, when terminating the scheduler pod, it sticks around until k8s ultimately sends it a SIGKILL after its grace period is up.

**What you expected to happen**:

The scheduler to exit after a single SIGTERM.

**How to reproduce it**:
The easiest way to reproduce it is to simply send a SIGKILL to your KubernetesExecutor scheduler pod:

```
$ kubectl exec {scheduler_pod_name} kill 1
```

Note, the pod stays running, while the logs report:

```
[2020-10-30 17:34:09,581] {scheduler_job.py:794} INFO - Exiting gracefully upon receiving signal 15
[2020-10-30 17:34:10,586] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,879] {process_utils.py:63} INFO - Process psutil.Process(pid=59, status='terminated', exitcode=0, started=
'17:33:23') (59) terminated with exit code 0
[2020-10-30 17:34:10,882] {process_utils.py:97} INFO - Sending Signals.SIGTERM to GPID 59
[2020-10-30 17:34:10,882] {scheduler_job.py:1329} INFO - Exited execute loop
```

Issue another `kill`:
```
$ kubectl exec {scheduler_pod_name} kill 1
```

And the pod will be restarted by k8s.

If you can't reproduce it, let me know and I'll try and get something running on kind.

**Anything else we need to know**:

The same behavior happens if you have an exception in the scheduler. An easy way to replicate it is to raise in a `pod_mutation_hook`.


airflow/jobs/scheduler_job.py
try:
            
            with redirect_stdout(StreamLogWriter(log, logging.INFO)), redirect_stderr(
                StreamLogWriter(log, logging.WARN)
            ), Stats.timer() as timer:
                
                settings.configure_orm()
                
                
                
                threading.current_thread().name = thread_name
                log.info("Started process (PID=%s) to work on %s", os.getpid(), file_path)
                dag_file_processor = DagFileProcessor(dag_ids=dag_ids, log=log)
                result: Tuple[int, int] = dag_file_processor.process_file(
                    file_path=file_path,
                    pickle_dags=pickle_dags,
                    callback_requests=callback_requests,
                )
                result_channel.send(result)
            log.info("Processing %s took %.3f seconds", file_path, timer.duration)
        except Exception:  
            
            log.exception("Got an exception! Propagating...")
            raise
        finally:
            
            
            settings.dispose_orm()
            result_channel.close()
    def start(self) -> None: