Comment 1 for bug 1770063

It's from openstack driver's requirement that we can't specify the vdev as required, since openstack driver doesn't know the vdev of ephemeral disk, it just know the format.
We can update doc to recommend customer to specify the vdev, format and mountpoint explicitly, since if customer want to configure these disk, he already the info for the disk he want to configure. Will update doc for it.