[DNM] Build venv into deployed source dir
This moves the python library installation to the fetch stage, and stores
inside the source dir so we rollback faster.
TODO:
- Why does the promote check only restart one service, and not ores-uwsgi?
- Need to plan deployment so either this or the puppet patch can be merged
without the other. Ideally, in either order.
Bug: T181071
Change-Id: Ibe44c2c3d7ec526a118a2b09a707616dd6cb3c05