Enable automatic "soft-restart" of workers on config changes

This way workers are restarted as soon as possible without
interfering currently running jobs when the worker config changes.

The restart of the services is triggered by a special systemd
unit. The salt configuration merely enables that unit. Hence this
change depends on the following upstream change:
os-autoinst/openQA#3666

Relevant tickets:
* https://progress.opensuse.org/issues/80910
* https://progress.opensuse.org/issues/80908#note-8

openqa/worker.sls

@@ -171,6 +171,10 @@ openqa-worker-auto-restart@{{ i }}:
       - pkg: worker-openqa.packages
       - stop_and_disable_all_workers
 {% endif %}
+
+openqa-soft-restart-worker@{{ i }}.path:
+  service.running:
+    - enable: True
 {% endfor %}
 
 openqa-worker.target: