[JENKINS-68363] - Fix dynamic DSL plugins handling in job DSL

[dhurum]

This commit fixes working with dynamic DSL plugins in job DSL integration code, namely in promotion actions processing.

Currently an attempt to use a dynamic DSL plugin in promotion actions causes Jenkins to raise an exception because dynamic DSL plugin nodes seem to have a different layout than a regular job DSL plugin.

See JENKINS-68363.

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• Pull request title represents the changelog entry which will be used in Release Notes. JIRA issue is a part of the title (see existing PRs as examples)
• Please describe what you did. Short summary for reviewers. If the change involves WebUI, add screenshots
• Link to relevant Jenkins JIRA issues or pull requests
• Test automation, if relevant. We expect autotests for all new features or complex bugfixes
• Documentation if relevant (new features). We want to use Documentation-as-Code, just put docs to README or to new Doc files
• Javadoc for new APIs. Any public/protected method is considered as API unless annotated by Restricted (docs)

CC

@oleg-nenashev @raul-arabaolaza

[dhurum]

Hey, @jtnord or @batmat, can you please take a look at this PR?

[jtnord]

Damn merged to early!

Generally please avoid using an assert for a non boolean primative.

If things fail, you just get a failure, rather it is better to use a higher level framework such as hamcrest so that when things fail you are told that "the needle does not exist in the haystack" (as the haystack can lead you directly to the cause/fix)