MXS-2011 Fix terminology and names

The variable storing the configuration index renamed from
'current_config' to 'config_index'.

In the code the same terminology is used as in the documentation.
"Default" is is used for referring to the default connections
(earlier "primary" was used) and the secondary connections are
referred to as "secondary" (earlier "alternative" was also used).

server/modules/routing/binlogrouter/blr.hh

@@ -838,7 +838,7 @@ struct ROUTER_INSTANCE: public MXS_ROUTER
     char              *set_slave_hostname;  /*< Send custom Hostname to Master */
     ROUTER_INSTANCE  *next;
     std::vector<ChangeMasterConfig> configs;/*< Available configs. */
-    int               current_config;       /*< The config currently being used. */
+    int               config_index;         /*< The index of the config currently being used. */
 };
 
 /** Master Semi-Sync capability */