`plugin_dir` does not have a default value

## What?

If `plugin_dir` is not set or omitted in `server.conf` there is no default value it falls back to upon server start, nor does there seem to be any sort of sanity check for it having a null/empty value.

The `bin_dir` config key has a default value of `/data` and if this is omitted it is apparent in the error log that Graylog is trying to load files from the `/data` directory. While ideally I'd like both values to have sanity checks for being null/empty on startup, I would settle for just having a default value for `plugin_dir` which I assume would produce a similar type of error, leading the Graylog admin in the right direction.

## Why?

Omitting `plugin_dir` produces a huge string of cryptic Java guice errors that don't lead the troubleshooter in any direction towrad the root cause, which is actually extremely simple. Having a sane default value (like /use/share/graylog-server/plugin which it's already set to in `server.conf`), even if not the value that points Graylog to the proper plugins it needs, will likely work 90% of the time. Having no default value set at all to fallback to will always work 0% of the time and also result in completely useless error log output.

## Your Environment

* Graylog Version: 5.1