Fix whitespace.
Add pull command to Centurion::Dogesty.

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.
Merge pull request #80 from toffer/dogestry-pull-multiple-hosts

Use the dogestry pull command to pull to multiple hosts.