Use self.addCleanup to schedule tmpdir cleanups

After prepare_runner_for_ssh was called, the tmpdirs created were
unreliably cleaned up. Use self.addCleanup to schedule the cleanup so
that users of the function don't need to cleanup manually.
Merge pull request #934 from Yelp/test_cleanup_master_ssh_root

Use self.addCleanup to schedule tmpdir cleanups