**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
#
#
import os
import sys
import token
from os.path import dirname
from shutil import copyfile, copytree, rmtree
from typing import List
from bowler import LN, TOKEN, Capture, Filename, Query
from fissix.fixer_util import Comma, KeywordArg, Name
from fissix.pytree import Leaf
from provider_packages.prepare_provider_packages import (
    get_source_airflow_folder,
    get_source_providers_folder,
    get_target_providers_folder,
    get_target_providers_package_folder,
)
def copy_provider_sources() -> None:
    """
    Copies provider sources to directory where they will be refactored.
    """
    def rm_build_dir() -> None:
        """
        Removes build directory.
        """
        build_dir = os.path.join(dirname(__file__), "build")
        if os.path.isdir(build_dir):
            rmtree(build_dir)
    def ignore_google_auth_backend(src: str, names: List[str]) -> List[str]: