Do not delete schedule if nova returns 404

Currently, if nova returns 404 qonos worker assumes the instance
does not exist. However, this inference might not be appropriate
in case of intermittent issues leading to potential customer data
loss.

Hence, the schedule will only be deleted through the nova extension.

Also, adds some logging.
Merge pull request #298 from komawar/dont-delete-schedules-on-404

Do not delete schedule if nova returns 404