When adding the Stroom service user, ensure its a system user

[burnalting]

As Stroom runs as a system service, it's user id should be a system one. In the documentation when the Stroom service user is created, it should be created as a system user.

This way standard user auditing postures from Linux auditd (e.g. monitoring file deletes) will not result in excessive auditd events during normal Stroom operation.