**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
"""
        Fixes to "odbc" providers package.
        Copies some of the classes used from core Airflow to "common.utils" package of the
        the provider and renames imports to use them from there.
        We copy helpers.py and change import as in example diff:
        .. code-block:: diff
            --- ./airflow/providers/google/cloud/example_dags/example_datacatalog.py
            +++ ./airflow/providers/google/cloud/example_dags/example_datacatalog.py
            @@ -37,7 +37,7 @@
                 CloudDataCatalogUpdateTagTemplateOperator,
             )
             from airflow.utils.dates import days_ago
            -from airflow.utils.helpers import chain
            +from airflow.providers.odbc.utils.helpers import chain
             default_args = {"start_date": days_ago(1)}
        """
        def odbc_package_filter(node: LN, capture: Capture, filename: Filename) -> bool: