Move fog unit tests to Core

They will still pass because they are using the old versions of the fog classes.
Use new `Core::Fog::ServiceInterface`
Use new `Core::Fog::Login` class
Use new `Core::Fog::ContentTypes` class
Move fog integration tests into Core
Move the top level fog credentials method to Core

Also move the unit test into `core/fog`. It was at the top level, not sure why.