Update nomad.service
Update nomad.service

While working on voxpupuli/puppet-nomad/pull/50 we observed that the released version of this file and the file in this repo are not in sync. This PR brings the systemd service file in sync with what is currently packaged into the rpm and deb packages as released in the HashiCorp repositories. I am not sure how/where to update the  [deployment guide](https://learn.hashicorp.com/tutorials/nomad/production-deployment-guide-vm-with-consul#configure-systemd)
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