Lockdown titan-server Cassandra config

This commit overrides CASSANDRA_INCLUDE, CASSANDRA_CONF, and
CASSANDRA_HOME environment variables when starting cassandra from the
titan-server distribution.  The objective is to avoid starting a
preexisting Cassandra daemon which might not have a suitable
configuration for use with Rexster and Titan.
Merge branch 'master' of github.com:thinkaurelius/titan