Rework the downtime schedule and cancel rpc calls.
Rework the downtime schedule and cancel rpc calls.

This changes a number of things:
- host: and hostgroup: identifiers can schedule host dowmtimes
- the cancellation key is now included in the comment in full
- the comment now includes the expression used to schedule the outage

marineam/nagcat,marineam/nagcat,marineam/nagcat