scheduled_task : SPIKE Implement prefetch / instances to report trigger changes more completely

When modifying an existing scheduled task trigger, values that are unmanaged are not reported in the "change to" message, and more configuration values may be reported in the "change from". This can be misleading to consumers of the module as it may seem like configuration values are being unset.

One way to fix this would be to copy unmanaged values from the read triggers to those defined in the manifest, though this could prove to be a tricky problem. There is no unique identifier for triggers, so matching them up on a task with multiple triggers may be complicated. Note that this would only impact Puppets first run against a task that previously was not under management, so may not be serious.

If a trigger is more completely filled in as part of prefetch / instances, then the code for triggers_same? in providers can likely be simplified. This also may mean that munge should be implemented to guarantee values are canonicalized (though that may be a separate effort / ticket).