Picketlink federation XML writer should not rely on the ordering in the ModelNode

AFAICS, FederationSubsystemWriter is relying on the ordering of the children of the root ModelNode when persisting the subsystem configuration. If one was to create a configuration using management operations from scratch, the operations would have to be ordered according to the order of the corresponding elements defined in the XSD schema. Which is not very practical and increases the chance of getting errors that could be avoided by making the writer persist the elements in the valid order.

E.g. in the current implementation if a management operation configuring key-store appeared after the one(s) configuring service-providers the resulting subsystem XML would be invalid according to the XSD.