Make image prefix configurable.

Adds a new parameter to docker.gradle `dockerImagePrefix` which specifies the PREFIX part in the following format: {REGISTRY}/{PREFIX}/{IMAGE}:{TAG}. This used to be hardcoded to "whisk", e.g. whisk/controller.

Implements a parameter for ansible to pull an image prefixed by "docker_image_prefix" (PREFIX in the following format): {REGISTRY}/{PREFIX}/{IMAGE}:{TAG}. Was previously hardcoded as "whisk".

Writes the parameter into whisk.properties and makes the invoker pick it up for spawning containers.

Add different tag in travis to flesh out problems.