title: Mongo service start or restart always fail
body: <p>I've installed mongodb then I've created a mongo service: </p>
  
  <pre><code> [Unit]
      Description=High-performance, schema-free document-oriented database
      After=network.target
  
      [Service]
      User=mongodb
      ExecStart=/usr/bin/mongod --quiet --config /etc/mongod.conf
  
      [Install]
      WantedBy=multi-user.target
  </code></pre>
  
  <p>But When I launch the service and then I check the status, I get always this error:</p>
  
  <pre><code>● mongodb.service - High-performance, schema-free document-oriented database
     Loaded: loaded (/etc/systemd/system/mongodb.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Mon 2017-04-24 13:08:55 UTC; 6min ago
    Process: 1094 ExecStart=/usr/bin/mongod --quiet --config /etc/mongod.conf (code=exited, status=48)
   Main PID: 1094 (code=exited, status=48)
  
  Apr 24 13:08:54 ip-172-31-37-163 systemd[1]: Started High-performance, schema-free document-oriented database.
  Apr 24 13:08:55 ip-172-31-37-163 systemd[1]: mongodb.service: Main process exited, code=exited, status=48/n/a
  Apr 24 13:08:55 ip-172-31-37-163 systemd[1]: mongodb.service: Unit entered failed state.
  Apr 24 13:08:55 ip-172-31-37-163 systemd[1]: mongodb.service: Failed with result 'exit-code'.
  </code></pre>