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
Assuming we expose the parseable time maintenance functionality to the jsonapi/diner as well, wouldn't we be better off doing this only once in check_entity.rb?  

My first thought was to add a create_maintenance like the find_maintenance and delete_maintenance, which would do the parsing, and then throw to create_scheduled_maintenance or create_unscheduled_maintenance.  Would this be a suitable option?