download_stream_optimized_image() does not support preprocessing of image

As implemented image_transfer.py:download_stream_optimized_image() is somewhat limited in that it takes the image_service/image_id and assumes it can use the image data obtain from the image server directly.
A recently use case has come up that requires some driver-specific processing of the glance image data to extract the actual stream optimized disk to be streamed to the server.
A more flexible alternative would be a function that takes a file-like object of the _actual_ data or a preprocessing callback.