Need push/pull for docker registry added to trcplgtool.

[mrjrieke]

In order to develop tierceron tools further, we need an enhancement to pkg/core/util/repository/dockercr.go (presently stubbed) to support pulling of images from a local docker registry running in a local docker swarm.

Begin by flushing out the stubbed code. This will get the pull working that is used by trcplgtool.

After that, you can look at implementing a push flag in order to give trcplgtool the ability to push images to the registry.