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`.

Basic test which confirms new inputs can be created and that messages
can be sent to a UDP-GELF input using `netcat`.

This test requires 4GB of RAM to avoid issues due insufficient
memory (please refer to `nixos/tests/elk.nix` for a detailed explanation of
the issue) for elasticsearch.

Also it's ensured that elasticsearch has an open HTTP port for communication
when starting `graylog`. This is a workaround to ensure that all services
are started in proper order, even in test environments with less power.
However this shouldn't be implemented in the `nixos/graylog` module as
this might be harmful when using elasticsearch clusters that require e.g.
authentication and/or run on different servers.

nixos/graylog: minor fixes, add test