Add systemd service file

[kwesthaus]

The service file used to control ratel with systemd was removed from the dgraph repo but has not yet been added here. This pull request just adds the service file to this repo, with the only modification being the new binary name.

---

This change is 

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[MichelDiz]

Sorry, but we are no longer building Ratel binary. It will only be in Docker image. Cheers.