Enhancements to scheduler priority functions

 - Modified the existing spreading priority to consider service pods explicitly
 - Added a new priority function to spread pods across zones
Another thing is, we wouldn't want this "zone" label to be hardcoded inside factory.go. After the plugin refactoring, this should be made a const inside the algorithm provider that defines the predicates/priorities.

If the above is acceptable, I'd say we let this be for now and change once the plugin refactoring PR merges.