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?)
That's a good point, and it raises a question: should a new developer (a contributor or new team member) by default run the ES tests by default (read: run *all* the tests by default), or spend so time wondering why some of the tests aren't running and cooking up a `local_settings.py` file just to enable the running of all the tests?

To me, it seems more logical to run all the tests by default, and if you want to "optimize" the test runs speed, then you disabled explicitely the ES tests. Thoughts?