Centralize timeout for waiting for servers to start.
    
    All service start/restart currently go through ipapython/platform so
    move the "wait for service to start" code there as well.
    
    A dictionary of known services and ports to wait on is defined in base.py
    This is referenced by the platforms by instance name to determine what
    to wait for. For the case of dirsrv if we get that as a plain name
    (no specific instance) it is assumed to be the main IPA service.
    
    https://fedorahosted.org/freeipa/ticket/2375
    https://fedorahosted.org/freeipa/ticket/2610