As image should have been already cached in CI, we shouldn't
download it again but just verify the checksum.
NOTE - at the time of this commit download.cirros.com is not working
so the cirros image is also switched to 0.4.0 which is present in
the infra CI image.
(cherry-picked from commit ad4841f70a)
Change-Id: Ib764c70e483513bec5bd97d4499e67beb4501826