treat docker-host more consistently when talking to dogestry

Currently, when Centurion runs the dogestry command to download an image, it passes an empty DOCKER_HOST environment variable; dogestry then defaults to tcp://localhost:2375, which doesn't work with newer versions of boot2docker that use a 192.168.59.x address.

This PR makes the Dogestry class handle the docker-host value a little more consistently; it removes the docker_host method that would also always return the default value (coincidentally, the same value dogestry uses).

@relistan @didip