Don't require runtime-related ENV variables for common development tasks

Created by: das-g

As a developer, I want to be able to perform common development tasks, such as creating migrations, without having to add my source into a docker container as a volume (in fact without having to use docker at all). Doing something like

virtualenv-3.4 .venv
. .venv/bin/activate
pip3 install -r osmaxx-py/requirements/local.txt
osmaxx-py/manage.py makemigrations

currently fails with django.core.exceptions.ImproperlyConfigured: Set the DJANGO_OSMAXX_CONVERSION_SERVICE_USERNAME environment variable

However, for running migrations this variable is completely irrelevant, so setting should not be required.

Note:

Fixing this shouldn't change that forgetting to set this variable in production causes an early failure to draw attention to that omission.