Improved idempotency
Improved idempotency

While specifying the image tag in the title works for getting the image
it breaks idempotency, so it is better to explicitly use the parameter.

Also, the exec had nothing to make it idempotent so I added a simple API
call to the docker registry, should work fine.

carthik/pltraining-classroom,joshsamuelson/pltraining-classroom,fnaard/pltraining-classroom,fnaard/pltraining-classroom,puppetlabs/pltraining-classroom,fnaard/pltraining-classroom,carthik/pltraining-classroom,binford2k/pltraining-classroom,samuelson/pltraining-classroom,samuelson/pltraining-classroom,carthik/pltraining-classroom,binford2k/pltraining-classroom,joshsamuelson/pltraining-classroom,joshsamuelson/pltraining-classroom,puppetlabs/pltraining-classroom,puppetlabs/pltraining-classroom,samuelson/pltraining-classroom,binford2k/pltraining-classroom