Metadata

One minor bug fix, config file readability, plus implementation of metadata IP and port in configuration.
As a follow-on, I hate that these are undecorated `Exception`s. We should really be doing:

```python
class ConfigError(Exception):
    """
    An error caused by invalid configuration.
    """
    pass
```

Adding this to a `calico.felix.exceptions` module is probably a good idea, we can then raise this where appropriate.