Offload copy_volume_to_image to ceph cluster

Offload rbd's copy_volume_to_image function from host to ceph cluster
could make full use of ceph's inherent data copy feature and the
hardware capacity of ceph storage cluster to expedite the volume data
copy speed, reduce the amount of data transmission and reduce the IO
load on cinder-volume host.

blueprint optimze-rbd-copy-volume-to-image

Change-Id: I5d43c06c3c9a8abde95dec0f8c6cb6c8c6a53e64
Merge "Offload copy_volume_to_image to ceph cluster"