**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/executors/local_executor.py
pid = os.fork()
        if pid:
            
            pid, ret = os.waitpid(pid, 0)
            return State.SUCCESS if ret == 0 else State.FAILED
        from airflow.sentry import Sentry
        ret = 1
        try:
            import signal
            from airflow.cli.cli_parser import get_parser
            signal.signal(signal.SIGINT, signal.SIG_DFL)
            signal.signal(signal.SIGTERM, signal.SIG_DFL)
            signal.signal(signal.SIGUSR2, signal.SIG_DFL)
            parser = get_parser()
            
            args = parser.parse_args(command[1:])
            args.shut_down_logging = False
            setproctitle(f"airflow task supervisor: {command}")
            args.func(args)
            ret = 0
            return State.SUCCESS
        except Exception as e:  
            self.log.error("Failed to execute task %s.", str(e))
        finally:
            Sentry.flush()
            logging.shutdown()
            os._exit(ret)  
            raise RuntimeError('unreachable -- keep mypy happy')
    @abstractmethod
    def do_work(self):