**Apache Airflow version**: 2.02


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.20.5

 **Environment**:

- **Cloud provider or hardware configuration**: AKS

**What happened**:

When using wasb for remote logging and backfilling a DAG, if the blob name already exists in the bucket, pods fails
```

> `Error in atexit._run_exitfuncs:
> Traceback (most recent call last):
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_upload_helpers.py", line 105, in upload_block_blob
>     **kwargs)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_generated/operations/_block_blob_operations.py", line 231, in upload
>     map_error(status_code=response.status_code, response=response, error_map=error_map)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/core/exceptions.py", line 102, in map_error
>     raise error
> azure.core.exceptions.ResourceExistsError: Operation returned an invalid status 'The specified blob already exists.'
> 
> During handling of the above exception, another exception occurred:
> 
> Traceback (most recent call last):
>   File "/usr/local/lib/python3.6/logging/__init__.py", line 1946, in shutdown
>     h.close()
>   File "/home/airflow/.local/lib/python3.6/site-packages/airflow/providers/microsoft/azure/log/wasb_task_handler.py", line 103, in close
>     self.wasb_write(log, remote_loc, append=True)
>   File "/home/airflow/.local/lib/python3.6/site-packages/airflow/providers/microsoft/azure/log/wasb_task_handler.py", line 192, in wasb_write
>     remote_log_location,
>   File "/home/airflow/.local/lib/python3.6/site-packages/airflow/providers/microsoft/azure/hooks/wasb.py", line 217, in load_string
>     self.upload(container_name, blob_name, string_data, **kwargs)
>   File "/home/airflow/.local/lib/python3.6/site-packages/airflow/providers/microsoft/azure/hooks/wasb.py", line 274, in upload
>     return blob_client.upload_blob(data, blob_type, length=length, **kwargs)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/core/tracing/decorator.py", line 83, in wrapper_use_tracer
>     return func(*args, **kwargs)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_blob_client.py", line 685, in upload_blob
>     return upload_block_blob(**options)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_upload_helpers.py", line 157, in upload_block_blob
>     process_storage_error(error)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_shared/response_handlers.py", line 150, in process_storage_error
>     error.raise_with_traceback()
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/core/exceptions.py", line 218, in raise_with_traceback
>     raise super(AzureError, self).with_traceback(self.exc_traceback)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_upload_helpers.py", line 105, in upload_block_blob
>     **kwargs)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/storage/blob/_generated/operations/_block_blob_operations.py", line 231, in upload
>     map_error(status_code=response.status_code, response=response, error_map=error_map)
>   File "/home/airflow/.local/lib/python3.6/site-packages/azure/core/exceptions.py", line 102, in map_error
>     raise error
> azure.core.exceptions.ResourceExistsError: The specified blob already exists.
> RequestId:8e0b61a7-c01e-0035-699d-4b837e000000
> Time:2021-05-18T04:19:41.7062904Z
> ErrorCode:BlobAlreadyExists
> Error:None
> `

```

**What you expected to happen**:

overwrite log file on backfills

**How to reproduce it**:

Run a trigger a dag run with remote logging wasb, delete dag and run again the same dag run.

tests/providers/microsoft/azure/log/test_wasb_task_handler.py
assert isinstance(self.wasb_task_handler.hook, WasbHook)
    @conf_vars({('logging', 'remote_log_conn_id'): 'wasb_default'})
    def test_hook_raises(self):
        handler = WasbTaskHandler(
            self.local_log_location, self.wasb_log_folder, self.container_name, self.filename_template, True
        )
        with mock.patch.object(handler.log, 'error') as mock_error:
            with mock.patch("airflow.providers.microsoft.azure.hooks.wasb.WasbHook") as mock_hook:
                mock_hook.side_effect = AzureHttpError("failed to connect", 404)
                
                handler.hook
            mock_error.assert_called_once_with(
                'Could not create an WasbHook with connection id "%s".'
                ' Please make sure that apache-airflow[azure] is installed'
                ' and the Wasb connection exists.',
                'wasb_default',
                exc_info=True,
            )
    def test_set_context_raw(self):
        self.ti.raw = True
        self.wasb_task_handler.set_context(self.ti)
        assert not self.wasb_task_handler.upload_on_close
    def test_set_context_not_raw(self):