Comment 1 for bug 1768107

To be clear, there are no in-tree filters that rely on RequestSpec.user_id, but people have out of tree filters that could have been using filter_properties['instance_properties']['user_id'] and the change in mitaka regressed them. It also seems OK to have RequestSpec.user_id since we have RequestSpec.project_id, and I could see it being useful to know which user created an instance when it's being moved if another user in the same project is moving the instance.