This change adds the `docker_service` resource for Docker swarm mode services. This
branches off some of the common elements (id, exists) into a `DockerObject` module along
with a utility function for parsing the image/repo string. That function was implemented
separately by `docker_image` and `docker_container`, now with a third resource, it made
sense to consolidate that into an included module. I used the most comprehensive
implementation. Existing classes had to be slightly modified for the genericization.
Signed-off-by: Matt Kulka <mkulka@parchment.com>
