Add a test to oslo.vmware test_image_transfer

This change adds unit tests of the following methods:
    * _start_transfer
    * download_stream_optimized_data
    * download_stream_optimized_image
    * upload_image

Change-Id: I2c99231f873b9bbe0d2d806966d7e69b10c80675
Merge "Add a test to oslo.vmware test_image_transfer"