@Joe mentioned it yesterday, the puppet master have been migrated to stretch, it makes sense to have the CI job to use Stretch as well.
A few patches have been made to prepare a CI container:
https://gerrit.wikimedia.org/r/#/c/integration/config/+/470419/
https://gerrit.wikimedia.org/r/#/c/integration/config/+/470574/
https://gerrit.wikimedia.org/r/#/c/integration/config/+/470834/
docker-registry.wikimedia.org/releng/operations-puppet:0.4.2 has Stretch.