broker: Correctly set up grpc leader proxy
broker: Correctly set up grpc leader proxy

Signed-off-by: Andrea Luzzardi <442ffe3ffbfd024afcf0b7f8b108d05bca3aae43@gmail.com>

deljuven/swarmkit,docker/swarmkit,docker/swarmkit,deljuven/swarmkit,sungwonh/swarmkit,sungwonh/swarmkit,fate-grand-order/swarmkit,aaronlehmann/swarmkit,allencloud/swarmkit,elifa/swarmkit,dperny/swarmkit-1,yongtang/swarmkit,cheyang/swarmkit,allencloud/swarmkit,yongtang/swarmkit,diogomonica/swarmkit,RenaudWasTaken/swarmkit,fate-grand-order/swarmkit,aaronlehmann/swarmkit,dperny/swarmkit-1,Noyan759/swarmkit,alyyousuf7/swarmkit,diogomonica/swarmkit,jlhawn/swarmkit,alyyousuf7/swarmkit,RenaudWasTaken/swarmkit,AliyunContainerService/swarmkit,cheyang/swarmkit,Noyan759/swarmkit,AliyunContainerService/swarmkit,jlhawn/swarmkit,elifa/swarmkit