Update TEMPEST_README.txt

Change I4bf4a02890a33c4034e4493d1763ed4019fdf46e introduced
using a venv to tempest for running the tests. However,
etc/TEMPEST_README.txt still listed it as a TODO. This
updates the readme with the new behavior.

Change-Id: I730b53996f2785b09f02d014f71e6b97d1e20e75