Only run flake8 on swiftclient code

In particular, this skips the "build" directory, which is prone to
containing stuff that doesn't comply with all the hacking checks. The
actual code we care about all lives in three places, so let's only
check those.

Change-Id: I4a3be59913ad85a5a9e91d254f4d3f4b0b1cc7b7

tox.ini

@@ -14,8 +14,7 @@ commands = python setup.py testr --testr-args="{posargs}"
 
 [testenv:pep8]
 commands =
-    flake8
-    flake8 bin/swift
+    flake8 bin/swift swiftclient tests
 
 [testenv:venv]
 commands = {posargs}