Fix default path in tools/with_venv.sh

Current default path for virtual environment in tools/with_venv.sh
is .venv, but official documentation uses tox, and actual path
for those following it will be .tox/pyXY. This patch tries to find
appropriate tox environment.

Change-Id: I78d8cbdf7cc31ccc27f3ef5c3c8cc0630b1b2eff