Prioritize nw endpoint which can provide both v4 and v6 connectivity
Prioritize nw endpoint which can provide both v4 and v6 connectivity

- when selecting sandbox's default gw

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

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