Data Node - Data Node 6.2 does not set correct default config file if started directly

## Expected Behavior
Data Node should start with the default config file located in `/etc/graylog/datanode/datanode.conf` if started directly via the binary

## Current Behavior
A wrong default value is set

## Steps to Reproduce (for bugs)
1. Install Data Node system package
2. Don't start via service, but directly using the binary `/usr/share/graylog-datanode/bin/graylog-datanode datanode` without specifying a config file using `-f`
3. observe startup error

## Context
Introduced by startup change refactoring

## Your Environment

* Graylog Version: 6.2-beta
* Java Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: