Update Ganga tutorial for nightlies

There are several recent changes that make it easier to run Ganga from the nighlies. We should adapt the tutorial accordingly.

1. lhcb-master is deployed on CVMFS with the highest priority. Being the most stable slot, we should recommend using it instead of lhcb-head.
2. All T1 sites should now have /cvmfs/lhcbdev.cern.ch mounted (after https://ggus.eu/?mode=ticket_info&ticket_id=151536 and https://ggus.eu/?mode=ticket_info&ticket_id=151537).
3. We can target only sites with /cvmfs/lhcbdev.cern.ch mounted with j.backend.diracOpts('setTag(["/cvmfs/lhcbdev.cern.ch/"])') (see LHCBDIRAC-890). MC samples should be replicated to T1 sites, and if not the case, we should check with lhcb-datamanagement@cern.ch.