MXS-2276 Use dynamic servers also for cluster check

Once the monitor has been able to connect to a clustrix node
and obtain the clustrix nodes, it'll primarily use those nodes
when looking for a Clustrix node to be used as the "hub".
With this change it is sufficient (but perhaps unwise) to provide
a single node boostrap node in the configuration file.

Some other rearrangements and renamings of functions has also been
made.

server/modules/monitor/clustrixmon/clustrixnode.cc

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2018 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2022-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "clustrixnode.hh"
+#include "clustrix.hh"
+
+bool ClustrixNode::can_be_used_as_hub(const MXS_MONITOR& mon)
+{
+    mxb_assert(m_pServer);
+
+    bool rv = Clustrix::ping_or_connect_to_hub(mon, *m_pServer, &m_pCon);
+
+    if (!rv)
+    {
+        mysql_close(m_pCon);
+        m_pCon = nullptr;
+    }
+
+    return rv;
+}