Switch to just using tox

Instead of requiring all the complicated
venv management logic that tox already provides
just use tox directly instead. This maps more
closely to what jenkins does, and how most other
python development occurs (which does not typically
rely on custom venv scripts).

Change-Id: Iff6cf0a7eb24ddd2ef65ddd723d0e4cb803b9bb9
Merge "Switch to just using tox"