(PDB-671) Enable JMX support for Jetty 9

This patch enables JMX support for exposing Jetty 9 metrics.

Signed-off-by: Ken Barber <ken@bob.sh>
I can add a test to ensure when its working easily enough, I can just grep. the jmx mbean list.

I'd need to change the way your `with-test-webserver` function works I think to take alternative configuration to test it when its disabled.