settings: Use environment variables by default

settings: Use environment variables by default

The Django documentation suggests using environment variables as a way
to allow devs to commit settings files via source control without
including confidential information therein [1]. As such, change the
provided 'production' settings to do this. This should promote best
practices while also ensuring provisioning tools like
'ansible-django-stack' [2] work as expected.

[1] https://docs.djangoproject.com/en/1.9/howto/deployment/checklist/
[2] https://github.com/jcalazan/ansible-django-stack

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>