compute: fix missing img_signature_uuid property

test_show_update_rebuild_list_server should use 
compute.certified_image_ref not only for creating the
instance (current state) but also for rebuilding it.
Otherwise the rebuild operation raises missing property error.

Closes-bug: 1839882

Change-Id: I79e32150837602a2e1b922e048e9fded165b89af
diff --git a/tempest/api/compute/servers/test_servers.py b/tempest/api/compute/servers/test_servers.py
index 76d65dd..1e3e966 100644
--- a/tempest/api/compute/servers/test_servers.py
+++ b/tempest/api/compute/servers/test_servers.py
@@ -209,7 +209,8 @@
                                        server['id'], 'ACTIVE')
 
         # Check rebuild API response schema
-        self.servers_client.rebuild_server(server['id'], self.image_ref_alt)
+        self.servers_client.rebuild_server(
+            server['id'], CONF.compute.certified_image_ref)
         waiters.wait_for_server_status(self.servers_client,
                                        server['id'], 'ACTIVE')