MXS-1775 Introduce MonitorInstance::configure() function

The configuring of the monitor instance is now performed in a
separate function. That is in preparation for the moving of the
start function to maxscale::MonitorInstance.

include/maxscale/monitor.hh

@@ -32,6 +32,8 @@ public:
 protected:
     MonitorInstance(MXS_MONITOR* pMonitor);
 
+    virtual void configure(const MXS_CONFIG_PARAMETER* pParams);
+
     virtual void main() = 0;
 
     static void main(void* pArg);