Not implicitly set vol_device_name to vda

Using a scsi decorated image with:
hw_disk_bus='scsi'
hw_scsi_model='virtio-scsi'

This solve the case where an instance is
launched with 'image' selected as boot
source with a new volume. This will result
in /dev/vda instead of /dev/sda as it
should.

Not specifying device name in
setFinalSpecBootImageToVolume() leaves the
decision to nova to determine it.

Example:
-------
VM boot volume without scsi meta data decoration:
Attached To
0a0cd660-7ce3-4033-9983-2e1099edc5f0 on /dev/vda

VM boot volume with scsi meta data decoration:
Attached To
91f50dbc-8bdf-4293-84ea-fc5df27b5ee4 on /dev/sda
--------

Note: This commit doesn't address cases for
where instances are launched using existing
volume and snapshot, this will involve more
work to migrate the code from BDMv1 to BDMv2.

Closes-Bug #1560965

Change-Id: I9d114c2c2e6736a8f1a8092afa568f930b656f09
(cherry picked from commit 0df60f4d)