Make avrorouter testing faster

The slave part of the binlogrouter replication can be ignored for
avrorouter tests. This will speed up the testing by skipping the
unnecessary restarting and configuration of slave servers.

maxscale-system-test/testconnections.h

@@ -442,6 +442,11 @@ public:
      */
     int start_binlog();
 
+    /**
+     * @brief Start binlogrouter replication from master
+     */
+    bool replicate_from_master();
+
     /**
      * @brief prepare_binlog clean up binlog directory, set proper access rights to it
      * @return 0