New builder to add a new docker template to all docker clouds

Hi there,

So this commit allows you to dynamically add a new template to all Docker clouds at build time. It is useful when you want to use an image built in a previous build step to provision a container in a following step.

Build scenario:
- Create a new image (using a dockerfile) and push it to a docker registry
- Add the image to all Dockers clouds
- Launch X builds in parallel (using the Parallel Test Executor Plugin for example), each running inside a container provisioned with the previously built image.