Add a possibility to run tests under callgrind

Flag 'use_callgrind' make all maxscale-system-test run Maxscale under Valgrind with --tool=callgrind option

maxscale-system-test/maxscales.h

@@ -20,7 +20,7 @@ public:
         READCONN_SLAVE
     };
 
-    Maxscales(const char *pref, const char *test_cwd, bool verbose, bool use_valgrind,
+    Maxscales(const char *pref, const char *test_cwd, bool verbose,
               std::string network_config);
 
     int read_env();
@@ -338,6 +338,12 @@ public:
      */
     bool use_valgrind;
 
+    /**
+     * @brief use_callgrind if true Maxscale will be executed under Valgrind with
+     * --callgrind option
+     */
+    bool use_callgrind;
+
     /**
      * @brief valgring_log_num Counter for Maxscale restarts to avoid Valgrind log overwriting
      */