Add support for Endpoint's preferred IPv6 address
Add support for Endpoint's preferred IPv6 address

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

mbanikazemi/libnetwork,jmzwcn/libnetwork,sainath14/libnetwork,amitkris/libnetwork,YujiOshima/libnetwork,docker/libnetwork,medallia/libnetwork,abhi/libnetwork,ityangchen/libnetwork,kolyshkin/libnetwork,mavenugo/libnetwork,anortef/libnetwork,WeiZhang555/libnetwork,vdemeester/libnetwork,coolljt0725/libnetwork,jimmycmh/libnetwork,curtiszimmerman/libnetwork,curtiszimmerman/libnetwork,docker/libnetwork,jimmycmh/libnetwork,tgraf/libnetwork,cpuguy83/libnetwork,fcrisciani/libnetwork,jmzwcn/libnetwork,mahak/libnetwork,anortef/libnetwork,msabansal/libnetwork,aboch/libnetwork_new,pyakpyak/libnetwork,tgraf/libnetwork,vdemeester/libnetwork,ityangchen/libnetwork,runcom/libnetwork,runcom/libnetwork,kolyshkin/libnetwork,mahak/libnetwork,sainath14/libnetwork,aboch/libnetwork_new,pyakpyak/libnetwork,fcrisciani/libnetwork,medallia/libnetwork,amitkris/libnetwork,YujiOshima/libnetwork,mbanikazemi/libnetwork,cpuguy83/libnetwork,coolljt0725/libnetwork,abhi/libnetwork,hackersdotmu/libnetwork,kolyshkin/libnetwork,hackersdotmu/libnetwork,mavenugo/libnetwork,mYmNeo/libnetwork,mahak/libnetwork,tmakita/libnetwork,abhi/libnetwork,vdemeester/libnetwork,msabansal/libnetwork,mYmNeo/libnetwork,allencloud/libnetwork,WeiZhang555/libnetwork,allencloud/libnetwork,tmakita/libnetwork,medallia/libnetwork,docker/libnetwork