Misc fixes all aimed at better exception handling.

1. Leverage utils.retry() inside utils.thread_coroutine().
2. Replace thread_coroutine() with tornado.concurrent.run_on_executor()
3. Add simple exception logger for threads.
4. bump tornado for https://github.com/tornadoweb/tornado/commit/8e7effdaefaddc5b61ba0aa6ff8e94f1422b6092
5. use python-rightscale retries branch
6. Retry on all exceptions rather than selected ones.
7. Allow the execute job to take longer (unit test)