Stack element in JGroups subsystem allows arbitrary attribute to be defined

<subsystem xmlns="urn:jboss:domain:jgroups:4.1">
    <channels default="ee" foo="bar">
	<channel name="ee" stack="udp"/>
    </channels>
    <stacks>
	<stack name="udp" foo="bar">
	    <transport type="UDP" socket-binding="jgroups-udp"/>
	    <protocol type="PING"/>
	    <protocol type="MERGE3"/>
	    <protocol type="FD_SOCK"/>
	    <protocol type="FD_ALL"/>
	    <protocol type="VERIFY_SUSPECT"/>
	    <protocol type="pbcast.NAKACK2"/>
	    <protocol type="UNICAST3"/>
	    <protocol type="pbcast.STABLE"/>
	    <protocol type="pbcast.GMS"/>
	    <protocol type="UFC"/>
	    <protocol type="MFC"/>
	    <protocol type="FRAG2"/>
	</stack>
    </stacks>
</subsystem>