Comment 5 for bug 1501435

In https://review.openstack.org/#/c/221525/ the intention was to remove bdm v2-only fields from the request to make novaclient bdm v1 boot work with nova API v2.1 schema validation. As Sean mentioned, the bug is that the volume_id field (a v1 and v2 field) was erroneously removed when "mapping_parts == 1"