Code review comment for lp:~rodsmith/checkbox/network-fixes

These changes were all to fix pre-existing problems identified by flake8. The temp file access is being left without context protection because protecting it would cause the whole test to fail if the temp file couldn't be created; but the point of storing the routing table in the temp file is to fix a minor annoyance (a missing default route after the test completes), and a failed test would be a bigger annoyance than that. Also, if the temp file can't be created, that implies that some other test -- probably a disk test -- would fail spectacularly, so the underlying problem can be left to that test for debugging.