Add support for docker exec to return cmd exitStatus

Closes #8703

Also made it so that when "docker exec" is run with -d (detached mode)
it'll print the id of the exec to the screen like "docker run -d" does
so people can query it via:
    curl http://.../exec/{id}/json

Signed-off-by: Doug Davis <dug@us.ibm.com>
do we need to print out the exec ID here ? As it is right now, the cli will not be able to do anything with it right ? Although i think it's nice to have if we expose the exec APIs in the cli.