Extract /containers operations into separate extension

There have been conversations recently about the scope of Magnum's API. Many people are divided about whether Magnum should function purely as a means to provision infrastructure, or as a more comprehensive solution that attempts to abstract over Container Orchestration Engine (COE) specific operations.

During these conversations, a compromise was reached: the COE logic of the /containers endpoint would remain, but as an optional extension. This provides users with the best of both worlds: the core API itself remains minimal, and users who want to wrap COE logic can do so by using the extension.

This is in keeping with many other OpenStack services which use extensions to offer functionality which is orthogonal to a main service API, e.g. Nova.

Because the following endpoints are already in the process of being removed:

/services
/pods
/rcs

This blueprint is concerned only with the /containers endpoint. All operations under this endpoint should be extracted from the core API and moved into an extension. Currently, these operations are:

- get container
- list containers
- create container
- update container
- delete container
- start container
- stop container
- pause container
- unpause container
- reboot container
- show container logs
- execute command in a container