MXS-836: Fix retry_on_failure not working

The service start retry mechanism mistakenly returned an error when a
service failed to start but a retry was queued. This caused MaxScale to
stop whenever a service failed to start.

server/core/config.c

@@ -135,6 +135,7 @@ static char *service_params[] =
     "ignore_databases_regex",
     "log_auth_warnings",
     "source", /**< Avrorouter only */
+    "retry_on_failure",
     NULL
 };