**Apache Airflow version**: 1.10.12


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): 1.16.9

**Environment**: 

- **Cloud provider or hardware configuration**: AWS
- **OS** (e.g. from /etc/os-release): 
- **Kernel** (e.g. `uname -a`): 
- **Install tools**: Docker image running in k8s Pods
- **Others**: Rancher-provisioned k8s clusters

**What happened**:

After configuring the latest version of the Elasticsearch backport provider as my log handler via `config/airflow_local_settings.py` resulted in an error on the webserver when trying to read logs from Elasticsearch

```
[2020-10-12 21:02:00,487] {app.py:1892} ERROR - Exception on /get_logs_with_metadata [GET]
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 121, in wrapper
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/flask_appbuilder/security/decorators.py", line 109, in wraps
    return f(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/decorators.py", line 56, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/db.py", line 74, in wrapper
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/views.py", line 733, in get_logs_with_metadata
    logs, metadata = _get_logs_with_metadata(try_number, metadata)
  File "/usr/local/lib/python3.7/site-packages/airflow/www_rbac/views.py", line 724, in _get_logs_with_metadata
    logs, metadatas = handler.read(ti, try_number, metadata=metadata)
  File "/usr/local/lib/python3.7/site-packages/airflow/utils/log/file_task_handler.py", line 194, in read
    logs[i] += log
TypeError: can only concatenate str (not "list") to str
```

Here is the relevant section of my customized `airflow_local_settings.py` file with the updated Elasticsearch handler from the backport provider:
```
...
    elif ELASTICSEARCH_HOST:
        ELASTICSEARCH_LOG_ID_TEMPLATE: str = conf.get('elasticsearch', 'LOG_ID_TEMPLATE')
        ELASTICSEARCH_END_OF_LOG_MARK: str = conf.get('elasticsearch', 'END_OF_LOG_MARK')
        ELASTICSEARCH_FRONTEND: str = conf.get('elasticsearch', 'frontend')
        ELASTICSEARCH_WRITE_STDOUT: bool = conf.getboolean('elasticsearch', 'WRITE_STDOUT')
        ELASTICSEARCH_JSON_FORMAT: bool = conf.getboolean('elasticsearch', 'JSON_FORMAT')
        ELASTICSEARCH_JSON_FIELDS: str = conf.get('elasticsearch', 'JSON_FIELDS')

        ELASTIC_REMOTE_HANDLERS: Dict[str, Dict[str, Union[str, bool]]] = {
            'task': {
                'class': 'airflow.providers.elasticsearch.log.es_task_handler.ElasticsearchTaskHandler',
                'formatter': 'airflow',
                'base_log_folder': str(os.path.expanduser(BASE_LOG_FOLDER)),
                'log_id_template': ELASTICSEARCH_LOG_ID_TEMPLATE,
                'filename_template': FILENAME_TEMPLATE,
                'end_of_log_mark': ELASTICSEARCH_END_OF_LOG_MARK,
                'host': ELASTICSEARCH_HOST,
                'frontend': ELASTICSEARCH_FRONTEND,
                'write_stdout': ELASTICSEARCH_WRITE_STDOUT,
                'json_format': ELASTICSEARCH_JSON_FORMAT,
                'json_fields': ELASTICSEARCH_JSON_FIELDS
            },
        }

        LOGGING_CONFIG['handlers'].update(ELASTIC_REMOTE_HANDLERS)
...
```

**What you expected to happen**:

Airflow's web UI properly displays the logs from Elasticsearch

**How to reproduce it**:
Configure custom logging via `config/airflow_local_settings.py` to `airflow.providers.elasticsearch.log.es_task_handler.ElasticsearchTaskHandler` and set the `logging_config_class` in `airflow.cfg`

When a task has been run, try to view its logs in the web UI and check the webserver logs to see the error above

provider_packages/refactor_provider_packages.py
for ch in node.post_order():
                if isinstance(ch, Leaf) and ch.value == "super":
                    if any(c.value for c in ch.parent.post_order() if isinstance(c, Leaf)):
                        ch.parent.remove()
        self.qry.select_subclass("BaseHook").modify(remove_super_init_call_modifier)
    def remove_tags(self):
        """
        Removes tags from execution of the operators (in example_dags). Note that those changes
        apply to example DAGs not to the operators/hooks erc. We package the example DAGs together
        with the provider classes and they should serve as examples independently on the version
        of Airflow it will be installed in. The tags are feature added in 1.10.10 and occasionally
        we will want to run example DAGs as system tests in pre-1.10.10 version so we want to
        remove the tags here.
        Example diff generated:
        .. code-block:: diff
            -- ./airflow/providers/amazon/aws/example_dags/example_datasync_2.py
            +++ ./airflow/providers/amazon/aws/example_dags/example_datasync_2.py
            @@ -83,8 +83,7 @@
             with models.DAG(
                 "example_datasync_2",
                 default_args=default_args,
            -    schedule_interval=None,  # Override to match your needs
            -    tags=['example'],
            +    schedule_interval=None,
             ) as dag:
        """
        def remove_tags_modifier(_: LN, capture: Capture, filename: Filename) -> None: