Adds support for a cluster-wide cassandra.in.sh file that is appeneded to the node specific one

This adds support for a common cassandra.in.sh file among all the nodes started by ccm. I needed this for integration with [cassandra-dtest](https://github.com/riptano/cassandra-dtest) for code coverage reporting.

If you can see a better way of doing this let me know, this has been tested by me for about a week with nothing blowing up.