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

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 +10,20 @@ set -e
 
 if systemctl -q is-active node-monitor.service; then
     echo "Disabling node-monitor as service"
+    systemctl stop node-monitor
+    # Wait until node-monitor is really stopped, maximum wait time 1 minute
+    for i in {1..60}; do
+        if ! systemctl -q is-active node-monitor; then
+            echo "node-monitor is stopped"
+            break
+        fi
+        sleep 1
+    done
+    if systemctl -q is-active node-monitor; then
+        echo "Failed to stop node-monitor within 1 minute"
+        exit 1
+    fi
     systemctl disable --now node-monitor
-    rm -f /etc/systemd/system/node-monitor.service
+    rm -fv etc/systemd/system/node-monitor.service
     systemctl daemon-reload
 fi