add 'boot from ebs' scenario test

test that boots from a snapshot of a volume backed instance
(instance that was booted from bootable volume that was created from image).

Recent changes in nova and cinder broke booting from EBS instance.
For example last change in cinder is here
(that broke booting) - https://review.openstack.org/#/c/182994/

Change-Id: I3915e2f04163568f758bb9e98bd5394af80aa431
Closes-Bug: #1485424
2 files changed