cainstance: Read CS.cfg for preop.pin in a loop
    
    As due to possible race conditions, the preop.pin might not be
    written in the CS.cfg at the time installer tries to read it.
    
    In case no value for preop.pin was found, retry until timeout
    was reached.
    
    https://fedorahosted.org/freeipa/ticket/3382
    
    Reviewed-By: Nathaniel McCallum <npmccallum@redhat.com>