MXS-1775 Make MariaDBMon non-dependent on stop() return value

To align it with the behavour or MonitorInstance::stop()

include/maxscale/monitor.hh

@@ -42,6 +42,18 @@ public:
      */
     int32_t state() const;
 
+    /**
+     * @brief Find out whether the monitor is running.
+     *
+     * @return True, if the monitor is running, false otherwise.
+     *
+     * @see state().
+     */
+    bool is_running() const
+    {
+        return state() == MXS_MONITOR_RUNNING;
+    }
+
     /**
      * @brief Starts the monitor.
      *