Fixed cluster_name change when installed with ubuntu package

There has been a discussion https://github.com/michaelklishin/cassandra-chef-cookbook/issues/92 related to some problems with custom cluster_name. Unfortunately solution provided in PR https://github.com/michaelklishin/cassandra-chef-cookbook/pull/105 doesn't help. What is missed is a 'nodetool flush' that sends that update to sstables.  
To make things simpler I have added missed command at the end of cqlsh call.

Tested on Ubuntu 14.04.1 LTS, and dsc 2.0.10