Ticket 47671 - CI lib389: allow to open a DirSrv without having to create the instance
    
    Bug Description:
    	DSAdmin allowed to bind to an already existing instance without creation of the instance.
    	Allocate() and open() require that a server-id is provided during Allocate.
    	This prevents to allocate a DirSrv? to bind to an existing instance.
    
    Fix Description:
    	Change Allocate so that SER_SERVERID_PROP is not mandatory.
    	Change Open so that if self.serverid is not defined it retrieves it from the instance (self.inst)
    
    https://fedorahosted.org/389/ticket/47671
    
    Reviewed by: Rich Megginson
    
    Platforms tested: F17/F19(jenkins)
    
    Flag Day: no
    
    Doc impact: no