Use resolveModelAttribute when reading the XTSSubsystemDefinition.DEFAULT_CONTEXT_PROPAGATION value for runtime use

Small code style thing. XTSSubsystemAdd.performBoottime is not using AttributeDefinition.resolveModelAttribute when reading the model for XTSSubsystemDefinition.DEFAULT_CONTEXT_PROPAGATION. This does no harm, as the attribute neither supports expression nor has a default value (see parent issue.) But it's better if we follow the KISS principle and apply the standard idiom without exception.