contrib/systemd: run as osmocom user

I have verified that with AmbientCapabilities=CAP_SYS_NICE, setting
scheduling policy as described in the manual still works as expected.

Related: OS#4107
Change-Id: Ibb83c231231b39dc6732c0f375aeb3b21f3938ef
diff --git a/contrib/systemd/osmo-mgw.service b/contrib/systemd/osmo-mgw.service
index 03f40ff..eaf84be 100644
--- a/contrib/systemd/osmo-mgw.service
+++ b/contrib/systemd/osmo-mgw.service
@@ -9,8 +9,11 @@
 StateDirectory=osmocom
 WorkingDirectory=%S/osmocom
 Restart=always
+User=osmocom
+Group=osmocom
 ExecStart=/usr/bin/osmo-mgw -s -c /etc/osmocom/osmo-mgw.cfg
 RestartSec=2
+AmbientCapabilities=CAP_SYS_NICE
 # CPU scheduling policy:
 CPUSchedulingPolicy=rr
 # For real-time scheduling policies an integer between 1 (lowest priority) and 99 (highest priority):