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
This package should be imported in plugin/pkg/scheduler/algorithmprovider/plugins.go. Also "AffinityProvider" can be added to the algorithmProviderNames var in plugin/pkg/scheduler/algorithmprovider/plugins_test.go to test that the provider is well formed.