Integrate with "ready-made" Docker images

[pm98zz-c]

We currently only can add additional services that are based on our base image (with SSH, ansible, ...). While it ensure consistency, it sometime is an overkill.
We should make it possible to add a service.XXX.yml that simply declare the use of another image. Using the docker_extras_args, and the networx related service only, this should be fairly do-able.
We need to introduce an additonal variable (managed: no or whatever we want to name it), that we can use in the vagrantfile to skip anything relating to SSH and provisioning.