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.cc

@@ -817,7 +817,7 @@ static MXS_ROUTER* createInstance(SERVICE *service, MXS_CONFIG_PARAMETER* params
      * automatic master replication start phase
      */
 
-    inst->current_config = 0;
+    inst->config_index = 0;
     rc = blr_file_read_master_config(inst);
 
     /*