Backward compatibility fix for resource-labels

[mPokornyETM]

fix #434

Upgrade to 1102 erased all resource labels when the user use .xml config file. It does not happens in case of CaC.
The magic is done by using of readResolve() method and 'repairing' the labels property.

To explain. Before the release 1102 are labels stored in String property. This was changed to List to improve performance, otherwise is necessary to split the String property every time, when you works with labels. On instances with many resources, where each use many of labels is the performance much better.

Why happens this issue? There are no tests to use .xml files. Only for JCaC. And I use only JCaC. That the reason, why I forgot to test the changes for classic .xml configurations.

Testing done

Tested manually:

• use 'old' .xml config file
• add ne resource and labels via config page and safe it
• reload jenkins with new .config file
• load CaC file

I checked the labels for every step as well and it looks good.

Proposed upgrade guidelines

N/A

Localizations

N/A

Submitter checklist

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