MXS-2481 Allow [un]block-commands to be overriden

In the case of Clustrix, there are other ports to block as well.

maxscale-system-test/mariadb_nodes.h

@@ -240,6 +240,18 @@ public:
     // Create the default users used by all tests
     void create_users(int node);
 
+    /**
+     * @param node Index of node to block.
+     * @return The command used for blocking a node.
+     */
+    virtual std::string block_command(int node) const;
+
+    /**
+     * @param node Index of node to unblock.
+     * @return The command used for unblocking a node.
+     */
+    virtual std::string unblock_command(int node) const;
+
     /**
      * @brif BlockNode setup firewall on a backend node to block MariaDB port
      * @param node Index of node to block