Geneate local vagrant box metadata.

Fixes #823.
For me, this command doesn't work.
```
(235)(noop-scheduler-235 ? =)[~/.../HybridLogic/flocker]$ vagrant box add --name vagrant/dev/flocker-dev.json
This command was not invoked properly. The help for this command is
available below.
```

Instead I used 

```
(235)(noop-scheduler-235 ? =)[~/.../HybridLogic/flocker]$ vagrant box add vagrant/dev/flocker-dev.json ==> box: Loading metadata for box 'vagrant/dev/flocker-dev.json'
    box: URL: file:///home/richard/projects/HybridLogic/flocker/vagrant/dev/flocker-dev.json
==> box: Adding box 'clusterhq/flocker-dev' (v0.1.2.418.g8d35b1f) for provider: virtualbox
    box: Downloading: /home/richard/projects/HybridLogic/flocker/vagrant/dev/flocker-dev-0.1.2-418-g8d35b1f.box
==> box: Successfully added box 'clusterhq/flocker-dev' (v0.1.2.418.g8d35b1f) for 'virtualbox'!
```