JPASubsystemAdd uses non-standard attribute value resolution

JPASubsystemAdd processes a couple attributes in non-standard ways, not using AttributeDefinition.resolveModelAttribute. The code handles the primary things resolveModelAttribute handles (default values and basic expression resolution) but it doesn't handle the full set of expression resolution capability WildFly supports.

The code also uses ModelNode.require to read the default-datasource attribute, which has the effect of forcing callers to set that attribute even though the resource description says it is optional and the runtime code can deal with a null value. To workaround this our standard config xml files are setting the xml attribute to the empty string which is unintuitive and shouldn't be needed.