[COOK-4140] Only notify when a rule is actually added

This fixes the `rule` resource to only notify whenever a rule is actually added.  This is done by using the `rule_exists?` method as a condition for convergence.  I also tried to improve the existence checks based on [COOK-3944](https://tickets.opscode.com/browse/COOK-3944) and [COOK-1070](https://tickets.opscode.com/browse/COOK-1070).

https://tickets.opscode.com/browse/COOK-4140