Fix the buildimages jobs: call the ensure-tox role
... and use its output.
tox is not available by default on the images anymore.
Conflicts:
roles/build-sahara-images-dib/defaults/main.yaml
roles/build-sahara-images-dib/tasks/main.yaml
tools/gate/build-images
The backport required some fixes due to the more simplified
structure of the build images role and script in the older
releases.
Change-Id: If5ea0ffdfc51860842c19f0a6f238127d42eb292
(cherry picked from commit 2d0c3a8b98)
(cherry picked from commit 0a9a43cb76)
(cherry picked from commit cdf83a23a0)
(cherry picked from commit 96abd08c28)