fix non-microsite SUBDOMAIN_BRANDING defaults

@chrisndodge we (and the vestigial edx.org hiera entry) do our `settings.COURSE_LISTINGS` like this:

```
  'COURSE_LISTINGS':
    'default':
      - 'Medicine/SP2L/2014'
```
so if there's no microsites config, the resultant subdomain should return `'default'`.  Otherwise all of the courses are displayed, instead of just those in `COURSE_LISTINGS`