Assert volume deletion in test_image_defined_boot_from_volume

Recently, we had a regression in the cinder api which resulted in the
inability to detach (and thus delete) a volume that had been created
from an image. This adds an assertion that the underlying volume has
been deleted in order to catch future regressions.

Related-Bug: #1768650

Change-Id: I5ef43b11e01f3c8980dcb364b23edda1b7af2859
1 file changed