NEGCACHE: Adding timeout to struct sss_nc_ctx
    
    It adds timeout of negative cache to handling
    struct sss_nc_ctx.
    
    There is one change in API of negatice cache:
     * int sss_ncache_init(TALLOC_CTX *memctx,
                           uint32_t timeout,  <----- new
                           struct sss_nc_ctx **_ctx);
    
    There is also one new function in common/responder:
    *  errno_t responder_get_neg_timeout_from_confdb(struct confdb_ctx *cdb,
                                                     uint32_t *ncache_timeout);
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2317
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>