Restrict JnlpSlaveRestarterInstaller to DumbSlave

[Vlatombe]

The exec was initially introduced to workaround memory leaks on long-running agents, it doesn't make sense to apply it to ephemeral nodes such as the ones managed by clouds.

Existing tests in JnlpSlaveRestarterInstallerTest still pass since they operate on static agents, unsure how I could introduce a cloud agent in test module.

See JENKINS-XXXXX.

Testing done

Proposed changelog entries

• SlaveRestarter implementations are now only installed on static agents. Use -Djenkins.slaves.restarter.JnlpSlaveRestarterInstaller.forceInstall=true to fall back to the previous behaviour in case of any issue.

Proposed upgrade guidelines

N/A

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[jglick]

unsure how I could introduce a cloud agent in test module

Possible with a mock-cloud dependency, though we try to avoid adding new test plugin deps to core. I suppose it would make sense to create a stripped-down version in jenkins-test-harness analogously to jenkinsci/jenkins-test-harness#79. In fact in jenkinsci/jenkins-test-harness#344 I did just that, though this would need to be opt-in to be mergeable.

[jglick]

Regarding a test case with a Cloud, this seems of pretty minimal value here—it is obvious by code inspection that the installer will not run on non-DumbSlaves. It is a bit less obvious that it will still run on DumbSlaves because it relies on Computer.getNode returning non-null at the moment onOnline is called.

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!