The sample of block_device_mapping_v2 contains uuid as a parameter
but there is not any explanation of the parameter.
This patch adds it to api-ref.
The following is for explaining the parameter from the code.
As the following code, source_type is "image" and api_dict["image_id"]
can be glance image-id on the sample case.
https://github.com/openstack/nova/blob/master/nova/block_device.py#L197
api_dict[source_type + '_id'] = device_uuid
The "image_id" is used for getting image metadata from Glance:
https://github.com/openstack/nova/blob/master/nova/compute/api.py#L1072
image_id = bdm['image_id']
image_meta = self.image_api.get(context, image_id)
Change-Id: I83badab07446c2b1e55cc3b29a3ba9476efc179d
9a5c20af7f