Fixing symlink to config dir for C* 2.1

For cassandra 2.1 the hard coded link to /usr/share/cassandra/default.conf/ won't work since it is never created and /etc/cassandra/default.conf/ is used instead. Fixed it by linking to default conf (/etc/cassandra/conf) which works for both versions.
In the medium term, it might be a better solution to modify cassandra.in.sh to set CASSANDRA_CONF making the hard coded symlink redundant...