Add cassandra.yaml overlay capabilities (can issue pull request now)

Adds a new file "conf/cassandra-overlay.yaml" that can contain any settings found in cassandra.yaml. Any settings, if found, override whatever is found in cassandra.yaml

A different overlay file can be specified using -Dcassandra.config.overlay=your_file_name
Overlay processing can be disabled with -Dcassandra.config.overlay.disable=true

Rationale: When administering cassandra nodes, I have found it quite common to want to distribute a common "golden" cassandra.yaml. This is challenging where you have a configuration value or two that needs to be modified per node. In this case, ops needs to know which lines of cassandra.yaml to ignore (because it's the same on all nodes) so that they can focus on what's uniquely configured for a particular node.

By specifying an additional overlay file, cassandra admins have the flexibility to decide what is configured on a per-node basis, and can make it extremely clear.

Source can be found in https://github.com/cmcconomy/cassandra/tree/config-overlay