Fix: Unshelving an instance uses original image

The driver uses the property image_ref to boot the
the instance.
This patch updates the image_ref property with the
shelved_image_id stored during the shelving.

Change-Id: I2304618bb5a995ea43f2a36069cfa0f3787c9792
Closes-Bug: #1257594
(cherry picked from commit 60455490c2c664dd41364ede29add64967a08844)
Merge "Fix: Unshelving an instance uses original image" into stable/havana