[META] Improve Hosting/Provision Services API

Our Services APIs are among of the stronger elements of the Aegir API. At least in that they're object-oriented, and will thus likely make upgrading to Drupal 8 and beyond somewhat easier. Our classes largely pre-date the introduction of traits into PHP, as well as Symfony and other frameworks. We should consider how to leverage these technologies to our advantage.

In particular, I'm thinking of the benefits we might see from #1260844: Refactor the web service to implement subservices and #881962: Use Cases: One server with multiple service instances on different ports.