Add tests with big backend

Basic tests can be executed with 15 machines Master/slave backend. Tests have label BIG_REPL_BACKEND Default template modified to support big backend. Tests temporaraly labeled as UNSTABLE to prevent their execution nightly
For big test maxscale.cnf is automatically generated for any number of nodes

maxscale-system-test/mariadb_nodes.h

@@ -482,6 +482,23 @@ public:
      */
     void limit_nodes(int new_N);
 
+    /**
+     * @brief cnf_servers Generates backend servers description for maxscale.cnf
+     * @return Servers description including IPs, ports
+     */
+    std::string cnf_servers();
+
+    /**
+     * @brief cnf_servers_line Generates list of backend servers for serivces definition in maxscale.cnf
+     * @return List of servers, e.g server1,server2,server3,...
+     */
+    std::string cnf_servers_line();
+
+    /**
+     * @brief cnf_server_name Prefix for backend server name ('server', 'gserver')
+     */
+    std::string cnf_server_name;
+
 private:
 
     bool check_master_node(MYSQL* conn);