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:
* Announce 1-hour downtime window.
* Pause the fetch score job.
* Take service down.* ~~Deploy new ORES code with both venvs, https://gerrit.wikimedia.org/r/#/c/392682/ ~~
* Deploy* Fix new ORES code with the venv, https://gerrit.wikimedia.org/r/#/c/392682/virtualenvs.
* Deploy service scripts to use the versioned venv: https://gerrit.wikimedia.org/r/#/c/392683/
* Reenable service and job.
* Smoke test
* Remove the unversionedshared venv from deployment targets.