Fix keystone warnings while building docs
When building the documentation via run_tests.sh, doc/source/conf.py
explicitly sets the DJANGO_SETTINGS_MODULE environment variable,
overriding any existing value that it already has. Furthermore, it sets
it to point to a settings file that does not use keystone v3, which
is expected by the tests (and is the source of the warning). Note that
when running unit tests via run_tests.sh, a different settings file is
supplied which DOES use keystone v3.
Change the doc/source/conf.py to only set DJANGO_SETTINGS_MODULE if it
is not already set. Change tox.ini and run_tests.sh to set
DJANGO_SETTINGS_MODULE to the same settings file used by the unit tests,
with keystone v3 support.