Flocker [1] is an open-source container data volume manager for
Dockerized applications.

This PR adds a volume plugin for Flocker.
The plugin interfaces the Flocker Control Service REST API [2] to
attachment attach the volume to the pod.

Each kubelet host should run Flocker agents (Container Agent and Dataset
Agent).

The kubelet will also require environment variables that contain the
host and port of the Flocker Control Service. (see Flocker architecture
[3] for more).

- `FLOCKER_CONTROL_SERVICE_HOST`
- `FLOCKER_CONTROL_SERVICE_PORT`

The contribution introduces a new 'flocker' volume type to the API with
fields:

- `datasetName`: which indicates the name of the dataset in Flocker
  added to metadata;
- `size`: a human-readable number that indicates the maximum size of the
  requested dataset.

Full documentation can be found docs/user-guide/volumes.md and examples
can be found at the examples/ folder

[1] https://clusterhq.com/flocker/introduction/
[2] https://docs.clusterhq.com/en/1.3.1/reference/api.html
[3] https://docs.clusterhq.com/en/1.3.1/concepts/architecture.html