Start using Pyflakes and Hacking

Instead of globally ignoring pyflakes and
hacking warnings, only blacklist those that trigger
very frequently so far, in order to clean them
up in followup commits. Fix and start gating
on the rest already.

Change-Id: Ied7c7250061e3bf379e8286e8ce3b9e4af817faf

tox.ini

@@ -26,6 +26,10 @@ commands = python setup.py testr --coverage --testr-args='{posargs}'
 downloadcache = ~/cache/pip
 
 [flake8]
-ignore = F,H
+# H302  import only modules
+# H303  no wildcard import
+# H306  imports not in alphabetical orde
+# H404  multi line docstring should start with a summary
+ignore = F403,F841,F812,F821,H302,H303,H306,H404
 show-source = True
 exclude = .venv,.tox,dist,doc,*egg