Add connectionbroker package
Add connectionbroker package

This package is a small abstraction on top of Remotes that provides a
gRPC connection to a manager. If running on a manager, it uses the unix
socket, otherwise it will pick a remote manager using Remotes. This
will allow things like agent and certificate renewal to work even on a
single node with no TCP port bound.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

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