neptune3-ui: Start neptune3 as the root user

By default systemd doesn't set a user when starting the service, but
because neptune3 needs some user specific directories we need to explicitly
set it to 'root'

Change-Id: Iffb042d6a7bc1b0bd121616b349c383fb9729a95
Reviewed-by: Vladimir Minenko <vladimir.minenko@pelagicore.com>
Reviewed-by: Samuli Piippo <samuli.piippo@qt.io>
(cherry picked from thud/4c70a657)
Fixes: AUTOSUITE-1143
Reviewed-by: Dominik Holland <dominik.holland@pelagicore.com>

meta-boot2qt/recipes-qt/automotive/neptune3-ui/neptune.service

@@ -3,6 +3,7 @@ Description=Neptune
 After=dbus.service dbus-session.service systemd-user-sessions.service
 
 [Service]
+User=root
 ExecStart=/usr/bin/appcontroller /opt/neptune3/neptune3-ui -r --dbus session -c am-config-neptune.yaml
 Restart=on-failure
 WorkingDirectory=/opt/neptune3