Allow for the running of arbitrary commands inside of a venv
so that we don't have to make tox envs for every blessed thing we want to
do.
Change-Id: Ia6f9e3cbeaef27ffe1e60b424c2a2d384e6bad0b
The tox.ini drives the current test suite in multiple virtual envs, so this
is a first step in trying to get multi-version testing.
Change-Id: I9c28ee11aa7bbe1b221a72e0ee8cc51d788f73c9