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>
Its also worth noting that if we do erase the metadata after the first GET that would mean its no longer idempotent.  Additionally, it means the client only gets one shot at it.  If there's a hiccup in the http flow and the response is lost then there's no way for the client to recover from this and get the info it may need.
Do we have other situations where we delete data after the first retrieval ?