MXS-2394 Accept substitute_variables

'substitute_variables' was not listed amongst the symbols that
are pre-parsed, which meant that it was always rejected.

server/core/config.cc

@@ -448,6 +448,7 @@ const char* config_pre_parse_global_params[] =
     CN_MAXLOG,
     CN_LOG_AUGMENTATION,
     CN_LOG_TO_SHM,
+    CN_SUBSTITUTE_VARIABLES,
     NULL
 };