Add support for nightlies base directory with "flavour" substructure

the current nightly builds are installed in /cvmfs/lhcbdev.cern.ch/nightlies/<slot>/<build_id> and only for the nightly flavour, but with the new system we have to install also testing and release flavours as the installation on CVMFS is used to build dependent projects in the slot, so the new system will deploy to /cvmfs/lhcbdev.cern.ch/nightlies/<flavour>/<slot>/<build_id>.

lb-run has to be ready to find installed nightly builds using both the old and the new convention and support the fully qualified slot build id as in lb-run --nightly testing/lhcb-head/123 ....