slapos_panel: use getPreferredHateoasUrl to generate the compute token

improve consistency with slaptool (configure this value in 1 place only)

master/bt5/slapos_panel/SkinTemplateItem/portal_skins/slapos_panel/Project_addSlapOSComputeNodeToken.py

@@ -15,9 +15,13 @@ if slapos_master_api is None:
 
 # This url is only used to call 1 (one) python script.
 # it could be whatever url able to reach erp5
-slapos_master_web_url = web_site.getLayoutProperty(
-    "configuration_slapos_master_web_url",
-    default=web_site.absolute_url()
+slapos_master_web_url = portal.portal_preferences.getPreferredHateoasUrl(None)
+if slapos_master_web_url is None:
+  return context.Base_redirect(
+    keep_items={
+      'portal_status_message': 'slapos_master_web_url is not configured',
+      'portal_status_level': 'error'
+    }
   )
 
 # XXX this url is HARDCODED on the client side