Validate the existence of the specified docker image before the cluster spins up

[jafreck]

currently, if a bad docker-repo is configured, the cluster will still be provisioned. Failure will only occur once the cluster tries to download the image. At this point the cluster will just hang in a bad state.

A check for the existence of the docker repo should be made before the cluster is created, and an error should be thrown if the image is bad.