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
One service per pod is definitely an assumption being made.

Technically, there is nothing stopping anyone from having multiple services point to the same pod. However, generally this is not true. Take a look at the discussion with @bgrant0607 
https://github.com/GoogleCloudPlatform/kubernetes/issues/1965#issuecomment-62197474.