Add related_bug for bug/1660878

The test test_reboot_deleted_server continued failing on the gate,
and the bug has been already fixed. This patch adds related_bug.

Change-Id: Id1aa7d01188697a5d85196a91dfc6a9a4cf4956f
Related-Bug: #1660878
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index 853d2ff..b22a434 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -178,6 +178,7 @@
                           self.client.rebuild_server,
                           server['id'], self.image_ref_alt)
 
+    @test.related_bug('1660878', status_code=409)
     @test.attr(type=['negative'])
     @decorators.idempotent_id('581a397d-5eab-486f-9cf9-1014bbd4c984')
     def test_reboot_deleted_server(self):