Return docker's root dir in docker -D info

This adds the docker daemon's root directory to docker info when running
in debug mode.  This allows the user to view the root directory where
docker is writing and storing state.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Merge pull request #9489 from crosbymichael/info-root

Return docker's root dir in docker -D info