cassandra/jenkins integration

Add a script that converts cassandra-stress output to an XML format that
is parseable by the Jenkins perfpublisher plugin.

The script is mostly a copy-paste of Avi's iperf script.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>