Change Sahara image name to sahara

Right now it is savanna. It will brake once
https://review.openstack.org/#/c/94638/1
is merged. So current CR should be merged at the same time
as the CR #94638.

Change-Id: I28c71747db8e52c4574891b9f8f78a5ea5a63427
Related-Bug: #1320245
Merge "Change Sahara image name to sahara"