Rename "cluster" files to "topology".

The Server Discovery And Monitoring Spec has replaced the term "cluster"
with "topology", since "cluster" in MongoDB terminology should be reserved
for sharded clusters. "Topology" describes a single server, replica set, or a
set of mongoses.

The next commit will replace "Cluster" names with "Topology".
Replace "Cluster" term with "Topology".

The Server Discovery And Monitoring Spec has replaced the term "cluster"
with "topology", since "cluster" in MongoDB terminology should be reserved
for sharded clusters. "Topology" describes a single server, replica set, or a
set of mongoses.

The previous commit replaced "cluster" with "topology" in file names. Here
I replace the term in class and variable names.
Auth doc should use term "deployment", not "cluster".

The term "cluster" is reserved for sharded clusters in MongoDB documentation.
This document should use the term "deployment" to mean any set of one or
more MongoDB servers.