Stub out Vcloud::Core::Fog::ServiceInterface

`Vcloud::Core::Fog::ServiceInterface` is instantiated when
[`Vcloud::Core::ApiInterface` is
initialised](https://github.com/gds-operations/vcloud-core/blob/a614e2263a76cdec04930e30573c9be2e3a9367c/lib/vcloud/core/api_interface.rb#L9).

Stub it out to prevent the unit tests from trying to connect to a real
vCloud Director instance.