fixed handing riak_control.user

Generated app.config doesn't include riak_control username and pass as userlist even if it's configured.

before fix

```
% grep riak_control app.2013.10.02.02.33.17.config
 {riak_control,[{enabled,true},{auth,userlist},{userlist,[]},{admin,true}]},
```

after fix

```
% grep -A 4 riak_control app.2013.10.02.10.09.40.config
 {riak_control,
     [{enabled,false},
      {auth,userlist},
      {userlist,[{"user","pass"}]},
      {admin,true}]},
```