Ability to choose docker repository on example workflows

[perithompson]

At the moment when trying the example workflows, there is no way of defining the docker registry that the applications and other images come from, with the docker pull rate limit this can cause the experiments to fail due to image pulls. Please provide a way to choose a custom repository for all images, not just the standard litmus images.

It would also be helpful to know where the docker images come from before running the experiments to check the cluster can reach all of the required images.