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
I didn't really give much thought on how this was supposed to be  done, so I just left it for the review. Important thing is that this is a minion label and should not be user modifiable (ones who create pods/replication controllers/services). This is something that the admin/installer is supposed to deal with and hence reserving a label for this purpose on the minions should be reasonable.