FAB-3: Fix Json output for empty network list

The generated pb.PeersMessage struct will be added "omitempty" tag automatically. So when
 marshal an empty PeersMessage, we get a "{}". Add a new tempoprary struct without
"omitempty" to replace PeerMesssage. Now we can get "{"Peer": []}"

Change-Id: I1fc310c951cf4e85a61660f2c45f700136dc5010
Signed-off-by: jiangyaoguo <jiangyaoguo@gmail.com>