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
Is there precedence for reserving a specific label key? If we do start treating certain labels *specially* we might want to reference them from a const where the special behavior is documented (possibly like we do for ports in [pkg/master/ports/ports.go](https://github.com/GoogleCloudPlatform/kubernetes/blob/master/pkg/master/ports/ports.go)) as well as documenting this behavior somewhere like the [labels doc](https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/labels.md).