Use the dogestry pull command to pull to multiple hosts.

Now that the `dogestry pull` command can pull to multiple docker hosts
(see https://github.com/dogestry/dogestry/pull/6), we can replace the previous two-step process
(`download_image_to_temp_dir()`, and then `upload_temp_dir_image_to_docker()`)
with a single call to `pull()`.

Also, since you can now pass the docker hosts to `dogestry pull` as command
line arguments, there's no longer a need for setting the `DOCKER_HOST` environment
variable before calling dogestry.

cc: @didip @relistan  @bryannewrelic