Introduce new slot for Gauss fast calo

Introduce a new slot called lhcb-gauss-fast to tests the fast calorimeter options.

This slot is identical to what is built by reference in lhcb-sim10 and in order for a merge request to be picked up in there the corresponding label should be used.

For the moment this is not deployed on cvfmsdev, as it can be done on request.

At the same time clean up the other slots a bit.