Add BNL replica to CVMFS_SERVER_URL

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>
Refactor release generation

This commits refactors the release generation to allow multiplatform
setups. It also starts versioning the vbox job descriptor.

Signed-off-by: Carlos Aguado Sanchez <carlos.aguado@epfl.ch>