Ticket 495 - 1.2.11 - plugin dn is missing from pblock
    
    Bug Description:  on 1.2.11 when setting the backend in the pblock we do not
                      set the plugin dn if its missing.  This causes inconsistencies
                      with internalModifiersname and internalcreatorsname.
    
    Fix description:  Set the plugin dn of the plugin if it is missing when setting the
                      backend in the pblock.
    
    https://fedorahosted.org/389/ticket/495
    
    Reviewed by: ?
    (cherry picked from commit d47fd4bc965e597c8d27368aa546d4012ce2546e)