Add flocker volume plugin

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

docs/user-guide/volumes.md

@@ -58,6 +58,7 @@ Familiarity with [pods](pods.md) is suggested.
       - [AWS EBS Example configuration](#aws-ebs-example-configuration)
     - [nfs](#nfs)
     - [iscsi](#iscsi)
+    - [flocker](#flocker)
     - [glusterfs](#glusterfs)
     - [rbd](#rbd)
     - [gitRepo](#gitrepo)
@@ -114,6 +115,7 @@ Kubernetes supports several types of Volumes:
    * `awsElasticBlockStore`
    * `nfs`
    * `iscsi`
+   * `flocker`
    * `glusterfs`
    * `rbd`
    * `gitRepo`
@@ -317,6 +319,21 @@ simultaneous readers allowed.
 
 See the [iSCSI example](../../examples/iscsi/) for more details.
 
+### flocker
+
+[Flocker](https://clusterhq.com/flocker) is an open-source clustered container data volume manager. It provides management
+and orchestration of data volumes backed by a variety of storage backends.
+
+A `flocker` volume allows a Flocker dataset to be mounted into a pod. If the
+dataset does not already exist in Flocker, it needs to be created with Flocker
+CLI or the using the Flocker API. If the dataset already exists it will
+reattached by Flocker to the node that the pod is scheduled. This means data
+can be "handed off" between pods as required.
+
+__Important: You must have your own Flocker installation running before you can use it__
+
+See the [Flocker example](../../examples/flocker/) for more details.
+
 ### glusterfs
 
 A `glusterfs` volume allows a [Glusterfs](http://www.gluster.org) (an open