In https://review.openstack.org/513200 the var specifying
the image was updated to a different structure and some
vars were removed. This updates the override to the new
structure.
Depends-On: I0b218fe276292e13dbb40ab38c9a167025d16e45
Change-Id: I4995fcac48889bd189cc1f25adb442b7a6637a46
(cherry picked from commit a016cb3444)