Issues with Defguard linux user during installation

[t-aleksander]

We've got two reports that this is not always working correctly:

1. There was a local defguard linux user before installation. After installing Defguard 2.0.0 via a deb file it got deleted.
2. The user was not created at all during the installation

We should verify if the user creation process during deb installation works as intended and possibly provide a troubleshooting guide, especially if we manage to find certain circumstances under which the process fails.