How to: How do I force replicas of a pod to split across different nodes?
Kubernetes by default does attempt node anti-affinity, but it is not a hard requirement, it is best effort, but will schedule multiple pods on the same node if that is the only way.

Learn more: 
http://stackoverflow.com/questions/28918056/does-the-kubernetes-scheduler-support-anti-affinity
http://kubernetes.io/docs/user-guide/node-selection/