Fixed entity metadata resolution.
    
    The current code assumes that an entity will always have a corresponding
    LDAPObject on the server, so it looks for the metadata in a fixed location.
    This assumption doesn't work for HBAC Test since it is a Command, not an
    LDAPObject, so the metadata has to be obtained from a different location.
    A new method get_default_metadata() has been added to allow each entity
    to find the metadata from the correct location.
    
    Ticket #388