When 'NetApp NFS Copy Offload' tool is configured to download Glance
images, these images are downloaded twice because the tool is doing
the job after Cinder has already done it.
This patch fixes the bug by executing the copy offload tool inside the
clone_image function instead of using the copy_image_to_volume.
Closes-bug: #1632333
Change-Id: I5c6ad150543213acfd0c78dbbdb1dc1584d22d26
