Fix packer DOCKER_HOST error

Packer currently throws a fatal error on OSX, when the DOCKER_HOST environment variable is set. As a lot of people use boot2docker, which uses that particular environment variable, it's not possible to use the Docker builder in that case. I changed the fatal error to a warning, as suggested in the issue below. I just started with Go, so there might be room for improvement :-)

See Issue #1476 for more information.