On send node envents, notify only if there are peers
On send node envents, notify only if there are peers

- Otherwise operation will unnecessarely block
  for five seconds.
- This is particularly noticeable on graceful
  shutdown of daemon in one node cluster.

Signed-off-by: Alessandro Boch <5bd2eb5b6f60921d9a2b9cb6a0a86bb947f3d361@docker.com>

medallia/libnetwork,medallia/libnetwork,abhi/libnetwork,mahak/libnetwork,allencloud/libnetwork,abhi/libnetwork,vdemeester/libnetwork,mahak/libnetwork,fcrisciani/libnetwork,ityangchen/libnetwork,docker/libnetwork,WeiZhang555/libnetwork,kolyshkin/libnetwork,mahak/libnetwork,WeiZhang555/libnetwork,kolyshkin/libnetwork,docker/libnetwork,medallia/libnetwork,vdemeester/libnetwork,docker/libnetwork,ityangchen/libnetwork,fcrisciani/libnetwork,vdemeester/libnetwork,abhi/libnetwork,allencloud/libnetwork,kolyshkin/libnetwork