**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`.


tests/jobs/test_scheduler_job.py
for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=False):
            detected_files.add(file_path)
        assert detected_files == expected_files
        ignored_files = {
            'helper.py',
        }
        example_dag_folder = airflow.example_dags.__path__[0]
        for root, _, files in os.walk(example_dag_folder):  
            for file_name in files:
                if file_name.endswith('.py') or file_name.endswith('.zip'):
                    if file_name not in ['__init__.py'] and file_name not in ignored_files:
                        expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(TEST_DAG_FOLDER, include_examples=True):
            detected_files.add(file_path)
        assert detected_files == expected_files
        smart_sensor_dag_folder = airflow.smart_sensor_dags.__path__[0]
        for root, _, files in os.walk(smart_sensor_dag_folder):
            for file_name in files:
                if (file_name.endswith('.py') or file_name.endswith('.zip')) and file_name not in [
                    '__init__.py'
                ]:
                    expected_files.add(os.path.join(root, file_name))
        detected_files.clear()
        for file_path in list_py_file_paths(
            TEST_DAG_FOLDER, include_examples=True, include_smart_sensor=True
        ):
            detected_files.add(file_path)
        assert detected_files == expected_files
    def test_adopt_or_reset_orphaned_tasks_nothing(self):