DIRAC v7r1

Dynamic instantiation of the FileCatalog plugins

FileCatalog is now instantiating plugins using ObjectLoader mechanism, which allows to provide plugins in extensions. One side effect is that SecurityPolicy plugins are dropped and the only one exisiting VOMSPolicy became VOMSSecurityManager. For those installations that used PolicyBasedSecurityManager, the configuration should be changed in order to use VOMSSecurityManager instead.