Allow non local mac-address.

Closes https://github.com/docker/docker/issues/9771, I also have a test I will PR to docker/docker to test this.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
I don't know if multicast matters or not. But it seems safer to split the check.