Don't use negative cache in netgroup lookup
    
    In responder a negative cache is used to indicate that the record has
    not been found by previous lookup. This approach is however not
    applicable for netgroup lookup because the design of their lookup is a
    little different.
    
    This patch removes some pieces of code working with negative cache,
    because they didn't fuction well. Instead a new flag has been added to
    the positive cache. This flag indicates if the record in the cache
    is a record of existing netgroup or it's just a placeholder.
    
    https://fedorahosted.org/sssd/ticket/820