Add support for special nightly build id 'latest' in lb-run and lb-dev

With the way new nightly builds are deployed, we cannot create the special symlink "latest" pointing to the latest build of a nightly slot.

This MR raises the handling of the special build id "latest" into lb-run (and lb-dev) lookup logic, making it possible to use again

lb-run --nightly lhcb-master/latest Moore/master

and

lb-dev --nightly lhcb-master/latest Moore/master

In must be noted that the lb-dev generated projects is now bound to the latest specific build id, which means that it does not change upstream slot every day.