Comment 54 for bug 1560965

I have instrumented horizon and compare the calls when creating a new instance with boot volume and when simply attaching a second volume.

Attaching a second volume doesn't pass any device value, which I make me think leaves the decision to cinder to give the volume name.

But when we launch a VM via horizon with a boot volume because of this 'vda' value being hardcorded, it send to cinder the device value 'vda'.

So setting vol_device_name to 'null' is IMHO the right thing to do as it'll be replicating what adding additional volume does already.