MXS-3301 Look for "xpand" to find out whether server is Xpand

We will continue to look for "clustrix" as well so that MaxScale
will continue to work with older releases. Clustrix was replaced with
xpand in all symbols.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -89,7 +89,7 @@ public:
         UNKNOWN,            /* Totally unknown. Server has not been connected to yet. */
         NORMAL,             /* A normal MariaDB/MySQL server, possibly supported. */
         BINLOG_ROUTER,      /* MaxScale binlog server. Requires special handling. */
-        CLUSTRIX            /* Clustrix server. Requires special handling. */
+        XPAND               /* Xpand server. Requires special handling. */
     };
 
     enum class BinlogMode