Always set image_uuid_alt in configure_tempest()

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
diff --git a/lib/tempest b/lib/tempest
index 4ba101f..2f62f6e 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -249,8 +249,8 @@
             1)
                 if [ -z "$image_uuid" ]; then
                     image_uuid=${images[0]}
-                    image_uuid_alt=${images[0]}
                 fi
+                image_uuid_alt=$image_uuid
                 ;;
             *)
                 if [ -z "$image_uuid" ]; then