Libcloud 544 gce metadata fix

This is a slightly improved version of the fix i submitted minutes ago which had some trace-generating code in it. Removed my own silly tests.
Actually, why can't we do something like this:

```python
metadata = {'items': []}
for key, value in ex_metadata.items():
    metadata['items'].append({'key': key, 'value': value})
```

This way, `ex_metadata` argument can still contain key named `items`...