Add ExecIDs to docker inspect

I need this for docker swarm because right now there is no way from the API to tell an execID is lined to a container.


Swarm has a mapping container <---> node so when Swarm receives a `/container/<container_id>/kill` request, we can use the `<container_id>` to find the right host to proxy the request to.

When Swarm receives a `/exec/<exec_id>/start` there is no way to find the right host right now.