Add backoffice certificate/ CA to Graylog

## What?
Take care that Graylog ALWAYS can connect and verify the Backoffice API certificate.

## Why?
It creates support issues and cases in the community where people create their own java keystore and do not have any "default" certificates given. This will make the remote checks for license fail and the system running without a license or failed remote checks.

It is always hard to debug as people tend to think they did the right but have some little steps lost.

This will only happen for Enterprise customers.

## Your Environment
* Graylog Version: 3.1