create_server: Couldn't specify the volume type when booting an image from volume

[Nayeong Kim]

I'm using a higher-level cloud layer for creating a server, and I want to specify the volume type with boot_from_volume option.

My desired `block_device_mapping_v2` is :
[{
    'source_type': 'image',
    'uuid': image_id,
    'boot_index': '0',
    'volume_size': 'volume_size,
    'destination_type': 'volume',
    'delete_on_termination': delete_on_termination,
    'volume_type':'my_volume_type'
}]

But ComputeCloudMixin.create_server does not take `volume_type` and it calls ComputeCloudMixin._get_boot_from_volume_kwargs which makes `block_device_mapping_v2` like:
[{
    'source_type': 'image',
    'uuid': image_id,
    'boot_index': '0',
    'volume_size': 'volume_size,
    'destination_type': 'volume',
    'delete_on_termination': delete_on_termination,
}]

-> No `volume_type` specified and the default one being used.

---------
conn = openstack.connect(cloud=CLOUD)

conn.create_server(
    image=image_id,
    boot_from_volume=True,
    volume_size=100,
    terminate_volume=True,
    name=hostname,
    nics=nics,
    flavor=flavor_id,
    availability_zone=availability_zone,
    userdata=userdata,
    meta=metadata
)
---------

To specify the volume type I can't use the high-level layer and should do like this :
---------
conn = openstack.connect(cloud=CLOUD)

conn.compute.create_server(
    block_device_mapping_v2=block_device,
    name=hostname,
    networks=[{'uuid': network_id, 'fixed_ip': available_ip}],
    flavorRef=flavor_id,
    availability_zone=availability_zone,
    user_data=encode_server_userdata(userdata),
    metadata=metadata
)
----------

It would be great if I can specify the `volume_type` for cloud layer's create_server().