Cannot enable MQTT authenticated mode for multiple users
## Alarm system
EVO192 v.7.70.1

## Environment
Rpi/docker

### Git branch
master

## Question
I'm trying to enable MQTT authenticated mode for multiple users, but the docker container fails at startup.

```
MQTT_CHALLENGE_SECRET = {'foo': 'bar'}
MQTT_CHALLENGE_ROUNDS = 10000
```

However, it starts regularly when using a single secret.
```
MQTT_CHALLENGE_SECRET = 'foo'
MQTT_CHALLENGE_ROUNDS = 10000
```

What's wrong with the first one?

## Logs (if required)
```
Attempting to load configuration from /etc/pai/pai.conf
Error parsing configuration: Invalid value type <class 'dict'> for config argument MQTT_CHALLENGE_SECRET. Allowed are: [<class 'str'>, <class 'NoneType'>]
```