qemu-img converter PoC

To enable the image convertion to raw in glance-api.conf use:

  convert_image_to_raw = True
  convert_image_to_raw_parallel = 5

Also convert_image_to_raw_path can be used to change the temporary
directory where image are converted.

Change-Id: I5f42beb7ca2e5e606c5c67823277aa750f7fe281