Since the upstream graylogctl script will prefer finding its java
executable based on JAVA_HOME, we now set this instead of PATH in
order to allow it to find the JRE. By setting it conditionally on it
not already being set, we allow selecting a different JRE at runtime.
We also explicitly use openjdk11, which supports the
UseConcMarkSweepGC option which graylog insists on using.
Until now the startup failed with an error like this:
```
com.github.joschi.jadconfig.ValidationException: Parent path /var/lib/graylog/server for Node ID file at /var/lib/graylog/server/node-id is not a directory
```
This happens since `graylog.service` ensures that `/var/lib/graylog`
exists, however it doesn't take care of the directory for
`cfg.nodeIdFile`.
