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**Apache Airflow version**: 2.2.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): 1.19 (server), 1.21 (client)

**Environment**:

- **Cloud provider or hardware configuration**: AWS EKS
- **OS** (e.g. from /etc/os-release): Docker image (apache/airflow:2.0.2-python3.7)
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**: 

`airflow` bin fails during configuration initialization (stack below). I see a similar issue reported here: https://github.com/apache/airflow/issues/13254, but my error is slightly different. 

```
  File "/home/airflow/.local/bin/airflow", line 5, in <module>
    from airflow.__main__ import main
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/__init__.py", line 34, in <module>
    from airflow import settings
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/settings.py", line 37, in <module>
    from airflow.configuration import AIRFLOW_HOME, WEBSERVER_CONFIG, conf  # NOQA F401
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 1098, in <module>
    conf = initialize_config()
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 860, in initialize_config
    conf.validate()
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 199, in validate
    self._validate_config_dependencies()
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 227, in _validate_config_dependencies
    is_sqlite = "sqlite" in self.get('core', 'sql_alchemy_conn')
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 328, in get
    option = self._get_environment_variables(deprecated_key, deprecated_section, key, section)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 394, in _get_environment_variables
    option = self._get_env_var_option(section, key)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 298, in _get_env_var_option
    return _get_config_value_from_secret_backend(os.environ[env_var_secret_path])
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 83, in _get_config_value_from_secret_backend
    secrets_client = get_custom_secret_backend()
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/configuration.py", line 999, in get_custom_secret_backend
    secrets_backend_cls = conf.getimport(section='secrets', key='backend')
NameError: name 'conf' is not defined
```
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:

`airflow` to correctly initialize the configuration.

**How to reproduce it**:
`airflow.cfg`
```
[core]
# ...
sql_alchemy_conn_secret = some-key

# ...
[secrets]
backend = airflow.contrib.secrets.aws_secrets_manager.SecretsManagerBackend
backend_kwargs = {config_prefix: 'airflow/config', connections_prefix: 'airflow/connections', variables_prefix: 'airflow/variables'}
```

**Anything else we need to know**:

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airflow/configuration.py
for (section, key) in self.sensitive_config_values:
            opt = self._get_secret_option(section, key)
            if opt:
                if not display_sensitive:
                    opt = '< hidden >'
                if display_source:
                    opt = (opt, 'secret')
                elif raw:
                    opt = opt.replace('%', '%%')
                config_sources.setdefault(section, OrderedDict()).update({key: opt})
                del config_sources[section][key + '_secret']
    def _include_commands(self, config_sources, display_sensitive, display_source, raw):
        for (section, key) in self.sensitive_config_values:
            opt = self._get_cmd_option(section, key)
            if not opt:
                continue
            if not display_sensitive:
                opt = '< hidden >'
            if display_source:
                opt = (opt, 'cmd')
            elif raw:
                opt = opt.replace('%', '%%')
            config_sources.setdefault(section, OrderedDict()).update({key: opt})
            del config_sources[section][key + '_cmd']
    def _include_envs(self, config_sources, display_sensitive, display_source, raw):