Potentially add simpler ways of specifying anti-affinity

[cheahjs]

#136 introduces the ability to specify the affinity field for the underlying etcd pods.

However, writing affinity rules requires an understanding of how the operator internally adds labels to pods. Given that for high availability the pods should be spread across multiple failure domains (nodes/availability zones), we might want to consider adding a simple API for specifying some topologyKey to spread pods across.