set key expiry properly for scheduled maintenance spanning current time

Grossly noticeable when the start time is relatively far in the past, and the duration long -- e.g. a 24 hour maintenance period starting 12 hours ago should expire in 12 hours but instead expires in 24. We were considering enforcing that the start time be in the future... maybe we should have.

Should also be cherry-picked for https://github.com/flapjack/flapjack/tree/maint/0.9