[v6r13] Clean-up and changes in the Storage classes

These are a number of changes in order to simplify the logic of the Storage classes:
- single place where the PFNs are constructed from LFNs ( StorageBase.getPfn() )
- multiple methods from specific Storage classes are moved to StorageBase
- StorageFactory creates Storage objects using a standard plugin mechanism ( ObjectLoader )
- allow to define VO specific path in the SE configuration
- check if StorageElement is allowed for the user's VO
- dirac-dms-show-se-status shows only SEs eligible for the user's VO
- fixes in the TestXXX classes
- suppressed ReplicaManager, MetadataCatalogClient
- some bug fixes and minor improvements
Those changes are still to be thoroughly tested.