MXS-2304 Clean up configuration serialization

The parameters are now written in the order they appear in the module
parameter definitions. Also enabled a previously disabled part in
server unit test.

server/core/test/test_server.cc

@@ -149,12 +149,9 @@ bool test_serialize()
     TEST(created->serialize(), "Failed to synchronize the copied server");
 
     /** Check that they serialize to identical files */
-    // TODO: Disabled for now. Enable once config parameters are kept in a well-defined order.
-    /*
     char cmd[1024];
     sprintf(cmd, "diff ./%s ./%s", config_name, old_config_name);
     TEST(system(cmd) == 0, "The files are not identical");
-     */
 
     return true;
 }