Make node startup more robust

The connection attempts to all nodes are done over a period of time to
cope with slowly starting servers.

maxscale-system-test/mariadb_nodes.h

@@ -153,6 +153,13 @@ public:
     int connect(int i);
     int connect();
 
+    /**
+     * Repeatedly try to connect with one second sleep in between attempts
+     *
+     * @return True on success
+     */
+    bool robust_connect(int n);
+
     /**
      * @brief Close connections opened by connect()
      *