Fix NPE when deleting a JClouds node.

[felfert]

When deleting a JClouds node that was restored from disk after a jenkins restart, hudson.model.Slave.drainLabelsToTrim() failed with an NPE, because previouslyAssignedLabels was NULL. Reason for that was: JCLoudsSlave provided a readResolve() which did not override. Therefore, hudson.model.Slave.readResolve() was never called when reloading from disk.

This PR adds an @Override and an invocation of super.readResolve() which initializes previouslyAssignedLabels.

Testing done

Manually create a JClouds node, then restart jenkins, then delete that node.

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