basic tox environnement

tox let you define virtual env and run whatever command you want
isolated. This change propose two environnement: py27 and flake8 (a
python linter).  You can invoke them with:

 tox -eflake8
 tox -epy27

Or simply by running `tox`.

Moved test dependencies to test-requirements.txt which is becoming a
standard.  The flake8 env has a single dependency (flake8) to avoid
installing all dependencies referenced in .txt files.

Change-Id: Id24f873c172d47cf31bfb66876c8b887322ecf45