XTSSubsystemDefinition.DEFAULT_CONTEXT_PROPAGATION should allow expressions and have default value

The XTS subsystem is deprecated so this is optional; I'm just recording it in case a decision is made to keep it for the long term.

XTSSubsystemDefinition.DEFAULT_CONTEXT_PROPAGATION is a basic boolean attribute so there is no reason it should not support expressions.

If it has no value the runtime service integration uses 'false' so that means false is the default value. The AttributeDefinition should reflect this so the read-resource-description output does.