Extend configuration to allow inline JGroups configuration and inheritance

<jgroups transport="org.infinispan.remoting.transport.jgroups.JGroupsTransport">
      <!-- Load external JGroups stacks -->
      <stack-file name="udp" path="stacks/udp.xml"/>
      <stack-file name="tcp" path="stacks/tcp.xml"/>
      <!-- Inline definition -->
      <stack name="mping">
         <TCP bind_port="7800" port_range="30" recv_buf_size="20000000" send_buf_size="640000"
              sock_conn_timeout="300" bundler_type="no-bundler"
              thread_pool.min_threads="0" thread_pool.max_threads="25" thread_pool.keep_alive_time="5000"/>
         <MPING bind_addr="127.0.0.1" break_on_coord_rsp="true"
                mcast_addr="${jgroups.mping.mcast_addr:228.2.4.6}"
                mcast_port="${jgroups.mping.mcast_port:43366}"
                ip_ttl="${jgroups.udp.ip_ttl:2}"/>

         <MERGE3/>
         <FD_SOCK/>
         <FD_ALL timeout="3000"
                 interval="1000"
                 timeout_check_interval="1000"
         />
         <VERIFY_SUSPECT timeout="1000"/>
         <pbcast.NAKACK2
                 use_mcast_xmit="false"
                 xmit_interval="100"
                 xmit_table_num_rows="50"
                 xmit_table_msgs_per_row="1024"
                 xmit_table_max_compaction_time="30000"/>
         <UNICAST3
                 xmit_interval="100"
                 xmit_table_num_rows="50"
                 xmit_table_msgs_per_row="1024"
                 xmit_table_max_compaction_time="30000"
         />
         <RSVP />
         <pbcast.STABLE stability_delay="200"
                        desired_avg_gossip="2000"
                        max_bytes="1M"
         />
         <pbcast.GMS print_local_addr="false"
                     join_timeout="${jgroups.join_timeout:2000}"/>
         <MFC max_credits="2M" min_threshold="0.40"/>
         <FRAG3/>
      </stack>
      <!-- Use the "tcp" stack but override some protocol attributes -->
      <stack name="mytcp" extends="tcp">
         <MERGE3 max_interval="20000"/>
      </stack>
      <!-- Use the "tcp" stack but replace the discovery -->
      <stack name="tcpgossip" extends="tcp">
         <TCPGOSSIP initial_hosts="${jgroups.tunnel.gossip_router_hosts:localhost[12001]}" stack.combine="replace:TCP_NIO2"/>
      </stack>
      <!-- Add a relay configuration using a previously declared stack to talk to the remote site -->
      <stack name="xsite" extends="udp">
         <relay site="LON">
            <remote-site name="NYC" stack="tcpgossip"/>
         </relay>
      </stack>
   </jgroups>