Make it easier to assign a flow definition attributes when registering it in a registry

Currently, there is no way to assign an flow definition arbitrary attributes when registering it using the convenient Spring 2.0 flow namespace flow:registry tag. When using this tag, flow definition attributes are only assigned by the flow itself. If this is not suitable, you must configure a XmlFlowRegistryFactoryBean manually. This ticket should investigate better ways to allow explicit flow attributes to be assigned.