Improvements to the layout of server.conf file

## What?

Reorganize the server.conf file to make it easier to fill in the required settings.

Either place all the required fields at the very beginning on the file so it's easy to fill out any paste over on other nodes. Or do like some other programs (I think opensearch or beats may do it this way) where you have a very basic config file and then a second .example file that contains ALL the options.

## Why?

I recently sat through a multi node deployment of Graylog and the configuration of the server.conf file felt a lot more challenging than it ideally could have. We only had console access, so could not ftp copies of the files between nodes. If you remove the file and recreate it the file permissions need to be fixed, some of the settings are not commented out so you can't really paste it all in the beginning etc.

On these three nodes it seriously took us about 10 minutes just to fill out the settings we needed and had to double check like 5 times that it was right.

## Your Environment

* Graylog Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: