Switch to flake8 from pep8.

 * flake8 supports more checks than pep8 (e.g. detection of
   unused imports and variables), and has an extension mechanism.
   A plugin to support automatic HACKING validation is planned.
 * See: http://flake8.readthedocs.org/

Change-Id: I1cba551fadf87f3dbc40a002736c1009e7b9d5b5