Locality for Replication Sets/Clusters

At times it is desirable to control where replicas in a replication set and instances in a cluster are created. In the case of read slaves, there may be a requirement to ensure that all replicas are on the same hypervisor, whereas for fault tolerance it may be desired that all replicas are created on different hypervisors. The same goes for clusters (mostly for anti-affinity here). Locality support would seek to address this issue.