Add JMX support

[gregw]

JMX support should be added as done in jetty-gae-compat-runtime, as this can assist debugging

[gregw]

This has a problem as jmxterm depends on jconsole.jar, which is not available in the headless jvm installed in this image. I will look for jmx clients that can work headlessly.

[dancek]

Would be great to integrate with Stackdriver Monitoring, which already supports JMX: https://cloud.google.com/monitoring/agent/plugins/jvm . Not sure if it's possible without modifying App Engine itself, considering the discussion at GoogleCloudPlatform/openjdk-runtime#37 .