### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Running `aws --endpoint-url=http://localhost:4566 iam get-user` returns:

```
Unable to parse response (syntax error: line 1, column 54), invalid XML received. Further retries may succeed:
b'<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">\n<title>500 Internal Server Error</title>\n<h1>Internal Server Error</h1>\n<p>The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.</p>\n'
```

### Expected Behavior

Return the actual data and not error 500.

### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

```
localstack start
```

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

```
aws --endpoint-url=http://localhost:4566 iam get-user
```


### Environment

```markdown
AWS CLI version: `aws-cli/2.4.6`
Localstack version: `0.13.2`
```


### Anything else?

Debug output:

```
2021-12-28T09:52:06.017:DEBUG:plugin.manager: instantiating plugin PluginSpec(localstack.aws.provider.iam:default = <function iam at 0x7fb864599310>)
2021-12-28T09:52:06.017:DEBUG:plugin.manager: loading plugin localstack.aws.provider:iam:default
2021-12-28T09:52:06.230:INFO:localstack.services.infra: Starting mock IAM service on http port 4566 ...
2021-12-28T09:52:06.231:INFO:localstack.services.motoserver: starting moto server on http://0.0.0.0:41753
2021-12-28T09:52:06.232:DEBUG:localstack.services.plugins: checking service health iam:41753
2021-12-28 09:52:06,267:API: Error on request:
Traceback (most recent call last):
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/werkzeug/serving.py", line 319, in run_wsgi
execute(self.server.app)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/werkzeug/serving.py", line 308, in execute
application_iter = app(environ, start_response)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/moto/server.py", line 239, in __call__
return backend_app(environ, start_response)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 2091, in __call__
return self.wsgi_app(environ, start_response)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 2076, in wsgi_app
response = self.handle_exception(e)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask_cors/extension.py", line 165, in wrapped_function
return cors_after_request(app.make_response(f(*args, **kwargs)))
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 2073, in wsgi_app
response = self.full_dispatch_request()
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 1518, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask_cors/extension.py", line 165, in wrapped_function
return cors_after_request(app.make_response(f(*args, **kwargs)))
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 1516, in full_dispatch_request
rv = self.dispatch_request()
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/flask/app.py", line 1502, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/opt/code/localstack/localstack/utils/aws/request_context.py", line 161, in convert_flask_to_httpretty_response_call
return convert_flask_to_httpretty_response_call_orig(*args, **kwargs)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/moto/core/utils.py", line 147, in __call__
result = self.callback(request, request.url, dict(request.headers))
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/moto/core/responses.py", line 203, in dispatch
return cls()._dispatch(*args, **kwargs)
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/moto/core/responses.py", line 313, in _dispatch
return self.call_action()
File "/opt/code/localstack/.venv/lib/python3.8/site-packages/moto/core/responses.py", line 403, in call_action
response = method()
File "/opt/code/localstack/localstack/services/iam/iam_starter.py", line 147, in iam_response_get_user
user = moto_iam_backend.users[user_name]
KeyError: 'default_user'
``` 

Appears to be coming from https://github.com/localstack/localstack/blob/master/localstack/services/iam/iam_starter.py#L147
os.environ.get("LAMBDA_CONTAINER_REGISTRY", "").strip() or DEFAULT_LAMBDA_CONTAINER_REGISTRY
)
LAMBDA_REMOVE_CONTAINERS = (
    os.environ.get("LAMBDA_REMOVE_CONTAINERS", "").lower().strip() not in FALSE_STRINGS
)
DATA_DIR = os.environ.get("DATA_DIR", "").strip()
TMP_FOLDER = os.path.join(tempfile.gettempdir(), "localstack")
if TMP_FOLDER.startswith("/var/folders/") and os.path.exists("/private%s" % TMP_FOLDER):
    TMP_FOLDER = "/private%s" % TMP_FOLDER
HOST_TMP_FOLDER = os.environ.get("HOST_TMP_FOLDER", TMP_FOLDER)
CACHE_DIR = os.environ.get("CACHE_DIR", os.path.join(TMP_FOLDER, "cache")).strip()
LS_LOG = eval_log_type("LS_LOG")
DEBUG = is_env_true("DEBUG") or LS_LOG in TRACE_LOG_LEVELS
DEVELOP = is_env_true("DEVELOP")
DEVELOP_PORT = int(os.environ.get("DEVELOP_PORT", "").strip() or DEFAULT_DEVELOP_PORT)
WAIT_FOR_DEBUGGER = is_env_true("WAIT_FOR_DEBUGGER")
USE_SSL = is_env_true("USE_SSL")
USE_SINGLE_REGION = is_env_true("USE_SINGLE_REGION")
TF_COMPAT_MODE = is_env_true("TF_COMPAT_MODE")
DEFAULT_ENCODING = "utf-8"
DOCKER_SOCK = os.environ.get("DOCKER_SOCK", "").strip() or "/var/run/docker.sock"
DOCKER_FLAGS = os.environ.get("DOCKER_FLAGS", "").strip()
DOCKER_CMD = os.environ.get("DOCKER_CMD", "").strip() or "docker"
LEGACY_DOCKER_CLIENT = is_env_true("LEGACY_DOCKER_CLIENT")
FORWARD_EDGE_INMEM = True
EDGE_BIND_HOST = os.environ.get("EDGE_BIND_HOST", "").strip() or "127.0.0.1"
EDGE_PORT = int(os.environ.get("EDGE_PORT") or 0) or DEFAULT_PORT_EDGE
EDGE_PORT_HTTP = int(os.environ.get("EDGE_PORT_HTTP") or 0)
EDGE_FORWARD_URL = os.environ.get("EDGE_FORWARD_URL", "").strip()
DOCKER_BRIDGE_IP = os.environ.get("DOCKER_BRIDGE_IP", "").strip()