this improves tox-venv to:
a.) consider ./.tox as the tox dir
useful if you run tox-venv from ~/bin and you're in cloud-init dir
with .tox there.
if no tox.ini and .tox are in this dir, fall back to using
the old look ${0%/*}/../.tox
b.) support '--list'
ie, running tox-venv adds support to tox-venv so you can list environments
explicitly
c.) outputs the list of available environments with --help
a '*' at the end indicates the environment appears presnt.
d.) any environment not present will be created with unless '--no-create'
e.) 4 spaces instead of 3 for indentation consistency with python
Example usage:
$ tox-venv docs python --version
Python 3.4.3+
Change-Id: I8a3d07c27fc3e4e41421e59ac5f47628a3612b83
37927ffd9c