Comment 3 for bug 1799892

OK so it looks like the root_provider_uuid should always be set, and if the resource provider is created without a parent_provider_uuid, then it is itself the root. But this is a data migration problem because for any existing providers created before queens ( https://review.openstack.org/#/c/377138/ ) wouldn't have either of those fields, and we didn't have any kind of online data migration to set the root_provider_id for existing providers.