Use `alert.cluster` when scheduling downtime

Use `alert.cluster` rather than `monitoring.management` when scheduling
downtime.

`monitoring.management` was renamed to `monitoring-1.management` in
fc9733. As of eb7284f, we have a `alert.cluster` which is aimed at
addressing one machine within a future group of monitoring boxes.
Merge pull request #36 from alphagov/rename-monitoring-host

Use `alert.cluster` when scheduling downtime