Fix docs build

With the updated docs build job, doc building is failing since
DJANGO_SETTINGS_MODULE is not set. Define it properly.

Change-Id: Idc43f59bf6d04705a30c02fd316f50706aacd4ef
(cherry picked from commit a368c4cdf6)

doc/source/conf.py

@@ -15,6 +15,9 @@
 import os
 import sys
 
+# Needs to be set for building documents without tox
+os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'ironic_ui.test.settings')
+
 sys.path.insert(0, os.path.abspath('../..'))
 # -- General configuration ----------------------------------------------------