Update for latest tripleo-image-elements changes.

Boot-stack is being split into smaller elements to deliver the three
use cases: seed cloud, undercloud, overcloud. As a result, the devtest
instructions have to update too.
Update for latest batch of changes.

devtest.md was out of sync due to changes to the
tripleo-image-elements repository that generalised the boot-stack
element and permitted deploying an undercloud.