Fix bug for `docker service ls`
Fix bug for `docker service ls`

fix bug for `docker service ls` error:
"Failed to retrieve backend list for service xxx (json: cannot
unmarshal object into Go value of type []client.sandboxResource)"

Signed-off-by: Zhang Wei <14976e959b2860fb06a3c1edbdb1945a4e11b8b5@huawei.com>

cpuguy83/libnetwork,tgraf/libnetwork,amitkris/libnetwork,curtiszimmerman/libnetwork,mYmNeo/libnetwork,jimmycmh/libnetwork,jmzwcn/libnetwork,vdemeester/libnetwork,kunalkushwaha/libnetwork,kolyshkin/libnetwork,kolyshkin/libnetwork,runcom/libnetwork,curtiszimmerman/libnetwork,jmzwcn/libnetwork,anortef/libnetwork,medallia/libnetwork,mahak/libnetwork,mavenugo/libnetwork,mapuri/libnetwork,mavenugo/libnetwork,AdoHe/libnetwork,vdemeester/libnetwork,abhi/libnetwork,mbanikazemi/libnetwork,medallia/libnetwork,sainath14/libnetwork,sanimej/libnetwork,sharmasushant/libnetwork,cpuguy83/libnetwork,ityangchen/libnetwork,hw-qiaolei/libnetwork,mahak/libnetwork,medallia/libnetwork,hw-qiaolei/libnetwork,coolljt0725/libnetwork,WeiZhang555/libnetwork,hackersdotmu/libnetwork,pyakpyak/libnetwork,msabansal/libnetwork,AdoHe/libnetwork,pyakpyak/libnetwork,abhi/libnetwork,tmakita/libnetwork,kunalkushwaha/libnetwork,abhi/libnetwork,sainath14/libnetwork,amitkris/libnetwork,mYmNeo/libnetwork,mahak/libnetwork,docker/libnetwork,ityangchen/libnetwork,allencloud/libnetwork,lindenlab/libnetwork,mbanikazemi/libnetwork,coolljt0725/libnetwork,hackersdotmu/libnetwork,frntn/libnetwork,allencloud/libnetwork,fcrisciani/libnetwork,aboch/libnetwork_new,YujiOshima/libnetwork,YujiOshima/libnetwork,tgraf/libnetwork,docker/libnetwork,msabansal/libnetwork,jimmycmh/libnetwork,WeiZhang555/libnetwork,runcom/libnetwork,tmakita/libnetwork,vdemeester/libnetwork,aboch/libnetwork_new,sharmasushant/libnetwork,junxu/libnetwork,junxu/libnetwork,anortef/libnetwork,frntn/libnetwork,mapuri/libnetwork,kolyshkin/libnetwork,sanimej/libnetwork,lindenlab/libnetwork,fcrisciani/libnetwork,docker/libnetwork