Adapt credentials plugin for cards

[timja]

See jenkinsci/credentials-plugin#984

Running full set of tests then I'll revert the local override so it can be merged

Testing done

Ran credentials plugin locally:

The two failures were weird frameworky issues, they passed on re-run.

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• Ensure that the pull request title represents the desired changelog entry
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• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[timja]

It would be better if the tests used ID for locating the credentials but since its not I was a bit worried that this could end up not being unique enough so I just made it more specific, can drop this though.