This lets us roll back much faster, since we won't need to rebuild the python environment.
Initial deployment needs to be done carefully, see notes in the operations/puppet patch. We need the puppet patch to be safe for both the new and old service scripts, and new and old path hierarchy.
Current migration plan:
* Deploy normal and versioned venvs to production, without changing requirements.txt: https://gerrit.wikimedia.org/r/#/c/392682/
* Change service scripts to use the versioned venv: https://gerrit.wikimedia.org/r/#/c/392683/
* Smoke test
* Remove the unversioned venv from deployment.