Support volume_type as a BDM parameter when boot instances

Currently when creating a new instance, user can call Nova API alongside
with "block_device_mapping_v2" which includes desired parameters like
" source_type, destination_type, volume_size " defined, to request nova
interacting with cinder to create a new volume and attach that volume to
the new instance.

However, the only volume property can be set by the user in the above
mentioned api is volume_size. On contrast, in cinder "create volume" api,
user can set both volume size and volume_type which is always used to
define the volume SLA/QoS. Therefore it is better for nova api to be
aligned with Cinder on this feature so that user can define the volume
SLA/QoS level when creating a volume associated with a given instance.