This makes tox use a simple wrapper around flake8 like
Nova, which can be told to restrict the check to only
files changed in the current command. This can be invoked
in a simple manner with 'tox -epep8 -- -HEAD'. Since most
commits only touch a handful of files, this will usually
be far faster than checking all source files.
To check an entire branch for bisectability it can be
automated via
git rebase -i master -x 'tox -epep8 -- -HEAD'
Change-Id: I75b1cfe3198b1217f8b25877714a4af47ae7069f
29b37065d7