Check pyflakes rules

This will catch unused vars and unused imports and similar code hygeine
items. Note this doesn't enable the other rules because we are trying to
ignore some and enable others and that doesn't seem doable with the
select/ignore lists.

Change-Id: If88de411bb18b395c3be492c0a6dccec50fe98f2

tox.ini

@@ -31,6 +31,6 @@ commands = {posargs}
 
 [flake8]
 ignore = E123,E125,H
-select = H231
+select = H231,F
 show-source = True
 exclude = .venv,.tox,dist,doc,build,*.egg