Implemented compute attach interfaces extension.

Attach interfaces API extension let to connect/disconnect running VMs to network ports. See http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-interface. Sorry I'm have no time to cover it with unit tests, but I'm check API methods in sandbox project.
I'm not sure about this annotation, when we constracted request for PUT method we pass
```javascript
{
    "interfaceAttachment": {
        "port_id": "ce531f90-199f-48c0-816c-13e38010b442"
    }
}
```
Is it mean that all except portId is nullable? I suppose that can be rewritten with payload annotation, after that all fields became not nullable.