Add a Docker::Service class for interfacing with Swarm-mode services

[danschmidt5189]

Overview

Add a Docker::Service class wrapping the Engine's Service-related endpoints:

• GET /services — List and filter services
• GET /services/{id} — Inspect a specific service
• POST /services/create — Create a service
• DELETE /services/{id} — Delete a service
• POST /services/{id}/update — Update a service
• GET /services/{id}/logs — Get or stream logs for a service

Considerations

1. Services relate to or depend on a number of other Swarm-scoped objects (Secret, Task, Config, …). It would make sense to implement those as well, and in some cases to provide helper methods linking a service to its related objects.
2. Since this depends on Docker running in Swarm mode (i.e. with docker swarm init) this will require significant pipeline and test-related changes.