Switch from pep8 to flake8/hacking.

Use flake8/hacking, as is standard in other OpenStack projects, but
disable new checks -- they will be enabled in later patches.

Flake8 catches many useful things
http://flake8.readthedocs.org/en/2.0/warnings.html, and hacking enforces
the OpenStack Style guidelines, including things like one import per
line and alphabetical imports in order to make merging easier.

Change-Id: Ib43f7a6790bc17c0b4efdda0a5f891b5a90303c4
Merge "Switch from pep8 to flake8/hacking."