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

tests/test_exc.py

@@ -24,6 +24,6 @@ FakeResponse = collections.namedtuple('HTTPResponse', ['status'])
 
 class TestHTTPExceptions(testtools.TestCase):
     def test_from_response(self):
-        """exc.from_response should return instance of an HTTP exception"""
+        """exc.from_response should return instance of an HTTP exception."""
         out = exc.from_response(FakeResponse(400))
         self.assertTrue(isinstance(out, exc.HTTPBadRequest))