[tox] Use generating environments
Drop the "env" option, which is unknown, and set a generative_ "envlist". Then
use conditional dependencies based on factors_ to avoid repeating environment
sections. The only singular one is the "cubicweb" environment because of its
"commands".
At some point, it'd be nice to find a way to have python version prefixes
(py27-hooks or py34-web) for environment names but I could not find a way to extract
the package name from such names.
.. _generative: http://tox.readthedocs.org/en/latest/config.html#generative-envlist
.. _factors: http://tox.readthedocs.org/en/latest/config.html#factors-and-factor-conditional-settings