Please consider merging branch `reboot` of repo `https://git.spwhitton.name/propellor`

- Factor out reboot code in `Propellor.Property.SiteSpecific.DigitalOcean` into `Propellor.Property.Reboot`
- Add `Propellor.Property.Reboot.toKernelNewerThan`.
- Add `Propellor.Property.SiteSpecific.Exoscale`

> [[done]]; all changes merged --[[Joey]]
