Enable automatic validation of many HACKING rules.

 * Add hacking to the tox build - a set of flake8 plugins that
   perform automatic validation of many HACKING.rst rules.
 * This patch configures hacking in the tox build and performs
   the mechanical cleanup required to allow the checks to pass.
 * See https://pypi.python.org/pypi/hacking

Change-Id: Ie9b3eb5627a9b6dd181f79225abae422e6cf3777
Merge "Enable automatic validation of many HACKING rules."