Change dnsmasq-restart cron delay from 30s to 15s

Task name already said "15s after reboot" but content had sleep 30.
Align content with name; faster recovery from systemd-resolved race at boot.

playbook/pxe_server_setup.yml

@@ -743,7 +743,7 @@
         dest: /etc/cron.d/dnsmasq-restart
         mode: '0644'
         content: |
-          @reboot root /bin/sleep 30 && /usr/bin/systemctl restart dnsmasq.service
+          @reboot root /bin/sleep 15 && /usr/bin/systemctl restart dnsmasq.service
 
     # --- Web Management App (Flask) ---
     - name: "Create webapp directory"