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)

dist/systemd/nomad.service

@@ -3,8 +3,6 @@ Description=Nomad
 Documentation=https://nomadproject.io/docs/
 Wants=network-online.target
 After=network-online.target
-StartLimitIntervalSec=10
-StartLimitBurst=3
 
 # When using Nomad with Consul it is not necessary to start Consul first. These
 # lines start Consul before Nomad as an optimization to avoid Nomad logging
@@ -13,14 +11,17 @@ StartLimitBurst=3
 #After=consul.service
 
 [Service]
+EnvironmentFile=/etc/nomad.d/nomad.env
 ExecReload=/bin/kill -HUP $MAINPID
-ExecStart=/usr/local/bin/nomad agent -config /etc/nomad.d
+ExecStart=/usr/bin/nomad agent -config /etc/nomad.d
 KillMode=process
 KillSignal=SIGINT
 LimitNOFILE=65536
 LimitNPROC=infinity
 Restart=on-failure
 RestartSec=2
+StartLimitBurst=3
+StartLimitInterval=10
 TasksMax=infinity
 OOMScoreAdjust=-1000