Kill a potential way to override CASSANDRA_INCLUDE

My last commit forced hardcoded values of CASSANDRA_HOME, _CONF, and
_INCLUDE when the titan-server distro's Cassandra instance was started
through titan.sh, but it left open the possibility of overridding
CASSANDRA_INCLUDE from the enviroment when executing titan-server's
cassandra script manually.  This commit hardcodes the environment in
the cassandra script.