Starting in Django 1.7, standalone scripts, such as a sphinx build
require that django.setup() be called first. See:
https://docs.djangoproject.com/en/1.8/releases/1.7/#standalone-scripts
It does not always necessary, but it is needed to avoid warnings
when creating autoindex.
Note that horizon no longer generates the autoindex to avoid warnings.
Drops setenv DJANGO_SETTINGS_MODULE in docs env in tox.ini.
DJANGO_SETTINGS_MODULE is configured in doc/source/conf.py,
so there is no need to pass envvar.
Add warning-is-error sphinx setting to avoid doc build warnings.
Change-Id: I2de87c0df05abf589c90fb3ad691322e1125c310
e965dc233e