Ticket 48263 - allow plugins to detect tombstone operations
    
    Bug Description:  if plugins do not want op operate on
                      tombstone operations there should be an easy
                      method to check taht it is a tombstone op
    
    Fix Description:  expose the OP_FLAG_IS_TOMBSTONE in the public slapi-plugin.h
    
    https://fedorahosted.org/389/ticket/48263
    
    Reviewed by: Rich, thanks