Enable per-provider models for openstack storage clients

This is my second take at making per-provider models work with openstack storage clients.

I like this better. 

Basically, any openstack storage client needs to keep a reference to the `File` and `Container` models used:

```javascript
this.models = {
  Container: require('../container').Container,
  File: require('../file').File
};
```

Then, anywhere the client uses a model, it's `this.models.File` or `this.models.Container`; More straight-forward I think.

- Fixes #269