Pass on_delete to XmlRpcLog.user explicitly
Pass on_delete to XmlRpcLog.user explicitly

As Django documentation mentions, on_delete will become a required
argument in Django 2.0.

Signed-off-by: Chenxiong Qi <b9f7aef17aabc89a75cac41179bbf54ad3f85119@gmail.com>

release-engineering/kobo,release-engineering/kobo,release-engineering/kobo,release-engineering/kobo