Systemd service units should set User/Group

[knorby]

With the current systemd service units deployed, mesos produced logs end up with "invalid-user" in the filename, and I believe I've seen that elsewhere. USER and GROUP aren't getting set on the mesos process's environment, which causes this behavior in glogs at least. Modifying to systemd unit to something like

[Service]
[...]
User=root
Group=root

fixes the problem. I'm noticing this problem on Ubuntu 16.04, where current log file names look like "mesos-{process}.{hostname}.invalid-user.log.{level}.{timestamp}". Making this change results in the user showing up as "root".