Add useInternal option for Rackspace/Openstack providers

This PR introduces the support for a `useInternal` property on `createClient`.

```Javascript
var client = require('pkgcloud').storage.createClient({
    provider: 'rackspace',
    username: 'your-user-name',
    apiKey: 'your-api-key',
    useInternal: true
});
```

Setting this option is explicit. If not found, your calls will time out.