[JENKINS-60434] Allow pipelines to continue running when preparing for shutdown

[ajkyffin]

Prevents pipelines from being paused when preparing for shutdown. No new parts of pipelines are scheduled to executors but parts already running can run to completion, allowing Jenkins to be restarted without any running jobs.

This is disabled by default and enabled by setting -Dorg.jenkinsci.plugins.workflow.cps.CpsThreadGroup.keepPipelinesRunningWhenQuietingDown=true.

This provides a solution for https://issues.jenkins.io/browse/JENKINS-60434.

Testing done

The default case is covered by existing tests in CpsFlowExecutionTest.java.

For the case with -Dorg.jenkinsci.plugins.workflow.cps.CpsThreadGroup.keepPipelinesRunningWhenQuietingDown=true:

• tested that pipelines are not automatically pause when preparing for shutdown
• tested manually pausing and resuming a pipeline
• tested manually pausing and resuming a pipeline while preparing for shutdown
• tested preparing for shutdown, allowing a pipeline to run to the end of it's current node section, and restarting Jenkins. The pipeline resumed at the next node section as expected.

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[jglick]

Likely functionally incomplete, deserves some test coverage, and ought to be an explicit option at the GUI (or CLI etc.) level rather than a system property. See discussion in Jira.