IcingaServiceSet: Delete host sets when deleting a set

This allows service sets to be deleted, including their children sets that
are assigned to hosts.

The UI forbids deleting those sets, but we can use it like this for CLI
and purging during sync.

library/Director/Objects/IcingaServiceSet.php

@@ -110,6 +110,19 @@ class IcingaServiceSet extends IcingaObject
         return $services;
     }
 
+    public function beforeDelete()
+    {
+        // check if this is a template, or directly assigned to a host
+        if ($this->get('host_id') === null) {
+            // find all host sets and delete them
+            foreach ($this->fetchHostSets() as $set) {
+                $set->delete();
+            }
+        }
+
+        parent::beforeDelete();
+    }
+
     public function onDelete()
     {
         $hostId = $this->get('host_id');