Cannot enable ralph service with systemd

### Steps to reproduce

- Restart VM in which ralph runs
- ralph is not automatically started
- `systemctl enable ralph` returns an error

### Expected behavior

We can use `systemctl enable ralph` so ralph is automatically started after boot.

### Actual behavior

`systemctl enable ralph` returns an error:

```
root@ralph:~# systemctl enable ralph.service
The unit files have no installation config (WantedBy, RequiredBy, Also, Alias
settings in the [Install] section, and DefaultInstance for template units).
This means they are not meant to be enabled using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
.wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
D-Bus, udev, scripted systemctl call, ...).
4) In case of template units, the unit is meant to be enabled with some
instance name specified.
```

### Environment

* Ralph version: 20211112.1
* Operating system: Ubuntu 18.04.6
* Method of installation: Deb package? Manual installation? Docker? Deb (APT repository)