Remove the need for a settings_local.py (bug 1050155)

fixes [bug 1050155](https://bugzilla.mozilla.org/show_bug.cgi?id=1050155)

This means that the `settings_local.py` won't be used automagically by `manage.py` anymore. If you want to use it, either:
- rename it to `local_settings.py` (imported, if it exists, by the new `settings.py` file), and make sure that you change the first line to be `from settings import *` (don't import from `lib/settings_base.py` or from `default.settings` directly, I'm not sure what would happen)
- use `DJANGO_SETTINGS_MODULE=settings_local` or the `--settings=settings_local` parameter when you run commands.

I've checked with @jasonthomas, this should be ok with his last changes (can you please give a look at this PR and confirm that it looks ok to you?)
Let's do that in another PR (with the merge of `sites/xxx/settings_base.py` and `sites/xxx/settings_addons.py`)