long flashcopy operation may block volume service

Storwize family uses flashcopy for snapshot or volume clone. The
volume delete has to wait until flashcopy finishes or errors out.
The _delete_vdisk() will poll volume FlashCopy status in a loop.
This may block volume serivce heartheat since it is in the same
. The solution is to use openstack FixedIntervalLoopingCall
to run the FlashCopy status poll in a timer thread.

The cinder volume mananger will resume delete operation for those
volumes that are in the deleting state during volume service startup.
Since Storwize volume delete may wait for a long time, this can cause
volume service to have long delay before it becomes available.
A greenpool is used to offload those volume delete operations.

Change-Id: Ie01a441a327e1e318fa8da0040ae130731b7a686
Closes-Bug: #1203152
(cherry picked from commit 7aa4f65a8c17aa037deff0f5b534ed694c17e62a)