fix(node-monitor): make sure it's really stopped

Try to prevent this error on some machines:

 Failed to disable unit: Unit file node-monitor.service does not exist.

The error is likely caused by a race condition

core/imageroot/var/lib/nethserver/node/update-core.d/80node_monitor

@@ -10,7 +10,9 @@ set -e
 
 if systemctl -q is-active node-monitor.service; then
     echo "Disabling node-monitor as service"
-    systemctl disable --now node-monitor
-    rm -f /etc/systemd/system/node-monitor.service
+    systemctl stop node-monitor || :
+    # The disable may fail if the update-core has already removed the service file
+    systemctl disable --now node-monitor || :
+    rm -fv /etc/systemd/system/node-monitor.service
     systemctl daemon-reload
 fi