Deployment namespacing - design review for 737

This demonstrates how namespacing might be implemented for #737.

* Gear will ripped out for docker, so I skipped implementation of that.
* Obvious division of work into issues is: volume namespacing, docker namespacing (i.e. IDockerClient), and adding it to orchestration.
* Note that unlike initial proposal the YAML config format knows nothing about namespacing.