Support adding arbitrary values to settings.py

This change makes it possible to add arbitrary additional values to ```settings.py```.

For example:
```
class{'puppetboard':
  # other parameters
  extra_settings => {
    NODES_FACTS = "['some_fact', 'some_other_fact']"
  },
}
```

It was written to work with [https://github.com/puppet-community/puppetboard/pull/108|@vjanelle's PR] to puppetboard which supports adding arbitrary fact values in the tables on the overview and nodes pages of puppetboard