Wait for all the processes to stop at exit

Now Onitu waits for Circus to kill all the processes before
exiting. Hence we give more chance to the processes to
do some cleanup before exiting.
Also, Onitu now logs that it's exiting.
Delete temporary sessions at exit

The DBs of temporary sessions are now deleted automatically.
It should be done by Escalator itself, but it's not really
possible as long as Escalator can handle several dbs.
Add Setup.clean to the tests

Setup.clean closes the reps, remove the JSON file and
delete the DB if a session is used.
Fix Python 3 compatibility. Add PY2 and PY3 helpers in utils.