Deployment of several compose file when running stack deploy

[efrecon]

Machinery automates (applications) stacks deployments through the applications key in the YAML file format. For the time being, only one YAML file can be deployed, while docker stack deploy supports having several YAML files that would be read on top of one another. While machinery supports extends (while docker does not), supporting several YAML files would permit a better one-to-one mapping to existing command-line tools and also facilitate breaking up big projects into smaller (logically regrouped) files.