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.
For all other extensions I see 
```java
Optional<? extends ...Api>
```
why defferent here?