System user cannot be created with a homedir

[Jakub Kuczys]

While it is not typical, system users can be created with a home directory by passing `-m/--create-home` flag to the command along with `-r/--system` flag. This cannot be done currently as `system: true` option will forcefully add the `-M` flag to the command. I think that there should be some way to do this other than manually calling `useradd`. I was expecting that if I put `homedir: /home/homeassistant` explicitly in the cloud-config, it will work, but it is not enough:
```
#cloud-config
users:
  - default
  - name: homeassistant
    gecos: Home Assistant
    system: true
    lock_passwd: true
    homedir: /home/homeassistant
```

[Chad Smith (chad.smith)]

Thanks for filing this issue and improving cloud=init. This is a confirmed gap in behavior and that override feels appropriate in my mind. Will run the idea past others to see if this is something we agree is fixable. It feels like defining a homedir with a system user indicates explicitly that you know what your intentions are.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3848