Add py3-dj21 tox env for Django 2.1
Add py3-dj21 tox env for Django 2.1

Django 2.1 has been released. It would be nice to have tox env
for testing with Django 2.1.

Note that this does not mean I lead Django 2.1 support.

Related-Bug: #1789046
Change-Id: Ic5f59e3c19480e01f5bfecbb0994121ee649388f

NeCTAR-RC/horizon,NeCTAR-RC/horizon,noironetworks/horizon,openstack/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon,openstack/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,openstack/horizon,noironetworks/horizon,noironetworks/horizon,noironetworks/horizon