Add DELETE_WHEN_FAILED variable

.gitlab-ci.yml

@@ -301,8 +301,10 @@ test_ood_image:
       fi
     - |
       if [ "$FAILED" = true ]; then
-        openstack server delete $NEW_INSTANCE_ID
-        openstack image delete $BUILT_OOD_IMAGE_ID
+        if [ "${DELETE_WHEN_FAILED-true}" = true ]; then
+          openstack server delete $NEW_INSTANCE_ID
+          openstack image delete $BUILT_OOD_IMAGE_ID
+        fi
         false
       fi
     - |