Configure tox to use flake8 instead of pep8 and pyflakes

Change-Id: I5c895c44543978c869aa706e3e1e5ba170180976
Signed-off-by: Rosario Di Somma <rosario.disomma@dreamhost.com>
Merge pull request #16 from rodis/feature/move-tox-to-flake8

Configure tox to use flake8 instead of pep8 and pyflakes