optimize the copy_image_to_volume method of sheepdogdriver
optimize the copy_image_to_volume method of sheepdogdriver

the current copy_image_to_volume method of sheepdogdriver can only
write the image file to local sheepdog, with this patch, it supports
write image file to remote sheepdog node.

Closes-Bug: #1509910
Change-Id: I5084ef2c048439ad8b9d6ae56ce8cccaadc26262

Nexenta/cinder,cloudbase/cinder,Hybrid-Cloud/cinder,dims/cinder,j-griffith/cinder,apporc/cinder,cloudbase/cinder,openstack/cinder,Hybrid-Cloud/cinder,mahak/cinder,eharney/cinder,Datera/cinder,eharney/cinder,openstack/cinder,Datera/cinder,bswartz/cinder,NetApp/cinder,scality/cinder,phenoxim/cinder,NetApp/cinder,ge0rgi/cinder,bswartz/cinder,scality/cinder,mahak/cinder,phenoxim/cinder,apporc/cinder,dims/cinder,j-griffith/cinder,Nexenta/cinder