Code review comment for lp://qastaging/~rconradharris/nova/backup_schedule_extension

Code looks good

Seems like something that might make people happy is if we load tempo the way we load a lot of the drivers, which is by flags. Then any service that wants to do something cron-like would have to provide the same API. The default/base driver effectively would do nothing, and then we don't need to explicitly reference Tempo in any way in the project