I just tried the following with Ocata (and Ceph as backend):
I added image properties hw_scsi_model=virtio-scsi
and hw_disk_bus=scsi
to an ISO image. Launching that ISO without attached disks results in /dev/sda device name in the partitioner. Then I attach new (empty) volumes to that instance, both are recognized as sdb and sdc.
EDIT:
Here's the command I'm using to create an instance from ISO attached with two volumes. Please note: this worked for me only with the already mentioned image properties for the ISO. Without them, the build fails because libvirt tries to allocate one volume as vda, the other as sda. This could be the result from the bug I created almost three years ago, which is still open.
nova boot --block-device source=image,id=<ISO_IMAGE>,dest=volume,size=5,shutdown=preserve,device=sda,bootindex=0 --block-device source=blank,dest=volume,size=5,shutdown=preserve,device=sdb,bootindex=1 --nic net-id=<NET_ID> --flavor 2 test1
This results in a running instance, trying to install from ISO, two volumes are attached: 