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 some long lineage, how about:

```ruby
case input
when /^more than/
  if input_timestamp < Time.now.to_i
    return maintenance_timestamp < input_timestamp
  else
    return maintenance_timestamp > input_timestamp
  end
when /^less than/
  # ...
end
```