Install jenkins flaky test plugin

[gregw]

We still have many flaky tests. We should install the Jenkins Flaky Test Handler to understand better which tests are flaky and which are not. We can then make hunting down those flaky tests a higher priority.

[olamy]

Started some work:

• setup rerunFailingTestsCount to 1 (if test failure the test is retry once) https://github.com/jetty/jetty.project/pull/11985
• setup Jenkins plugin https://github.com/jetty/jetty.project/pull/11984
• setup launchable (https://www.launchableinc.com/docs/features/unhealthy-tests/) to collect data https://github.com/jetty/jetty.project/pull/11983

[olamy]

done