Issue #3441587, by ernanesabino: Adding missing configuration to schema.yml file

Closes #3441587

config/schema/adv_varnish.schema.yml

@@ -29,6 +29,9 @@ adv_varnish.cache_settings:
         purger_maintenance_mode:
           type: boolean
           label: 'Prevent Varnish purge while in Maintenance Mode'
+        grace:
+          type: string
+          label: 'Grace'
     available:
       type: mapping
       mapping: