Don't attach stdin and stdout when in detach mode

This is primarily to make it work with Swarm, which checks that
AttachStd{in,out,err} is false when creating containers.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #763 from bfirsh/pass-through-detach-to-create-container

Don't attach stdin and stdout when in detach mode