Docker Clustering: Design proposal.

Authors: @aluzzardi and @vieux.

The goal and scope of this change is to allow docker to manage a cluster
of docker hosts.

The target audience for clustering is modeled after Docker itself:
developer and devops first then enterprise later. Enterprise requires
additional features such as authentication, ACLs, auditing and tooling
which will come at a later time.

The system is designed to handle workloads in the same way as Docker: it
can run both long-running and one-off tasks. Batch processing can be
built on top of the API using the one-off primitive.

The architecture is based on an evented model where the master queries
the registered slaves within the system and checks the current state of
the slaves against the requested state by the user.  It reconciles any
differences and updates the cluster state with the required changes.

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>
Signed-off-by: Victor Vieux <vieux@docker.com>
Any thoughts on being able to specify conditions for an *image* and allow specifying them in a Dockerfile (@cpuguy83 has a proposal for specifying "dependencies" in Dockerfiles, I don't have the issue number at hand, but possibly this is useful)

One problem is that the constraints in this proposal are user defined; for interoperability, pre-defined constraints should exist, so that I can pull an image of the hub, configure my nodes (using the pre-defined constraints) and have the cluster automatically distribute my containers based on those constraints.