topology_creator fails for larger networks

The problem is that

```
  54                node_names.append("%c%i" % (chr(97 + i), j))
```

results in

```
1004             <node binary="/home/lo/native/RIOT/examples/default/bin/native/default.elf" name="<80>1     " type="riot_native"/>
```

which cannot be parsed anymore:

```
xml.parsers.expat.ExpatError: not well-formed (invalid token): line 1004, column 94
```
