Exclude service images

[shyr]

In docker swarm mode, keeping some images used by docker service could be better for those reasons.

1. There is highly chance that the images is needed when service is updated and container should be run on the node.
2. For some case like "drain node", docker rescheduling containers without sending registry auth. If there is no images on the node received message to run container by docker swarm, and the image need registry auth, the service would be fail.

This PR propose that if environment variable 'EXCLUDE_SERVICE_IMAGE' is true, docker-gc skip to clean images used by docker service.