**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
([21, 22, 24, 26], 1, 1, "1d", "30m", "star"),  
            ([21, 22, 24, 26], 1, 1, "1d", "30m", "grid"),  
            
            ([9, 9, 9, 9], 1, 5, "1d", "None", "no_structure"),  
            ([9, 9, 9, 9], 1, 5, "1d", "None", "linear"),  
            ([21, 12, 12, 12], 1, 5, "1d", "@once", "no_structure"),  
            ([22, 13, 13, 13], 1, 5, "1d", "@once", "linear"),  
            ([21, 22, 24, 26], 1, 5, "1d", "30m", "no_structure"),  
            ([22, 24, 27, 30], 1, 5, "1d", "30m", "linear"),  
            ([22, 24, 27, 30], 1, 5, "1d", "30m", "binary_tree"),  
            ([22, 24, 27, 30], 1, 5, "1d", "30m", "star"),  
            ([22, 24, 27, 30], 1, 5, "1d", "30m", "grid"),  
            
            ([9, 9, 9, 9], 10, 10, "1d", "None", "no_structure"),  
            ([9, 9, 9, 9], 10, 10, "1d", "None", "linear"),  
            ([84, 27, 27, 27], 10, 10, "1d", "@once", "no_structure"),  
            ([94, 40, 40, 40], 10, 10, "1d", "@once", "linear"),  
            ([84, 88, 88, 88], 10, 10, "1d", "30m", "no_structure"),  
            ([94, 114, 114, 114], 10, 10, "1d", "30m", "linear"),  
            ([94, 108, 108, 108], 10, 10, "1d", "30m", "binary_tree"),  
            ([94, 108, 108, 108], 10, 10, "1d", "30m", "star"),  
            ([94, 108, 108, 108], 10, 10, "1d", "30m", "grid"),  
        ]
    )
    def test_process_dags_queries_count(
        self, expected_query_counts, dag_count, task_count, start_ago, schedule_interval, shape
    ):
        with mock.patch.dict(
            "os.environ",
            {