Add return code if the end unscheduled maintenance was successful (closes #595)

In end_unscheduled_maintenance, the last command executed is @redis.zadd("#{@key}:unscheduled_maintenances", duration, um_start). This returns false, as this key already exists, having been added in create_unscheduled_maintenance.

Why does this key exist, and is it intended to be used somewhere?

The actual work here is done by @redis.del("#{@key}:unscheduled_maintenance"), which returns one if successful. I suggest we make the success of this the return of the function, where it returns a proper boolean if the maintenance was deleted.