When there is only a single image, configure_tempest()
needs to always set image_uuid_alt the same as image_uuid,
else it will fail trying to determine the size of the
flavor to use for it later in the function.
Introduced by [0], and subsequent change did not fix it.
[0] https://review.opendev.org/c/openstack/devstack/+/886795
Change-Id: Ibfe99ff732570dbd415772c5625f43e35b68c871
Related-bug: #2028123
e261bd809e