Add swarm.Swarm to swarm.Info…
Add swarm.Swarm to swarm.Info…

… in order to display Swarm information in the `/info` endpoint.

Signed-off-by: Vincent Demeester <87e1f221a672a14a323e57bb65eaea19d3ed3804@sbr.pm>

vdemeester/engine-api,docker/engine-api,thaJeztah/engine-api,jwhonce/engine-api