Add Jenkins cleanup thread.

It searches for outstanding builds that haven't reported back
or otherwise can't be found by Jenkins, and marks them complete
with a result of "LOST".  This should help to avoid deadlocks
where Zuul waits forever to hear back from Jenkins after missing
a notification.

Add pyflakes to tox.ini.

Change-Id: I26d3fbf375e82e224448ec3337f9cc97deeccd56
diff --git a/tox.ini b/tox.ini
index 2ddaee1..7ec660e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,10 @@
 [tox]
-envlist = pep8
+envlist = pep8, pyflakes
 
 [testenv:pep8]
 deps = pep8
 commands = pep8 --repeat --show-source zuul zuul-server setup.py
+
+[testenv:pyflakes]
+deps = pyflakes
+commands = pyflakes zuul zuul-server setup.py