Add maintenance to the CLI (closes #562, #563)

As part of this, I've also:

- Updated create_*scheduled_maintenance to allow for an input string to be parsed by Chronic.  This has no effect on existing code.
- Fixed end_unscheduled_maintenance to return a boolean when the unscheduled maintenance is ended (Also done in #596)
- Moved the times in the tests to global variables
- Had rubocop fix indenting/spacing on tests
That's correct.  At present, entities that have checks that have received events are available via Flapjack::Data::EntityCheck.find_all, and are available under current_checks:#{entity}.  Scheduled maintenance only sets #{@key}:unscheduled_maintenance, and the two are never crosschecked.