`-zone` parameter flag isn't used for disks

I recently tried to run the `networkinterfacenaming` suite in a different zone (the default is `us-central1-a` and I tried `us-east4-a`) but got the following error in the logs: ``` <failure message="" type="Failure"><![CDATA[step "create-vms" validation error: cannot create instance in project "us-central1-a" with disk in zone "us-east4-a": "c3metal"]]></failure> ``` I haven't had a chance yet to properly investigate but after a quick check it looks like perhaps the chosen zone isn't being passed to `bootdisk.Zone` (via `daisy.Disk{}`)?