### 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
TRUE_STRINGS,
)
load_start_time = time.time()
class Directories:
    """
    Holds the different directories available to localstack. Some directories are shared between the host and the
    localstack container, some live only on the host and some only in the container.
    Attributes:
        static_libs: container only; binaries and libraries statically packaged with the image
        var_libs:    shared; binaries and libraries+data computed at runtime: lazy-loaded binaries, ssl cert, ...
        cache:       shared; ephemeral data that has to persist across localstack runs and reboots
        tmp:         shared; ephemeral data that has to persist across localstack runs but not reboots
        functions:   shared; volume to communicate between host<->lambda containers
        data:        shared; holds localstack state, pods, ...
        config:      host only; pre-defined configuration values, cached credentials, machine id, ...
        init:        shared; user-defined provisioning scripts executed in the container when it starts
        logs:        shared; log files produced by localstack
    """
    static_libs: str
    var_libs: str
    cache: str
    tmp: str
    functions: str
    data: str
    config: str
    init: str
    logs: str
    
    default_bind_mounts = ["var_libs", "cache", "tmp", "data", "init", "logs"]
    def __init__(