Update supported keystone versions
Update supported keystone versions

index.rst did not list keystone v3 support

Change-Id: I4a0305c424c994c3beda184bbdc53fecb9c08586
Closes-Bug: #1474990

dan1/django_openstack_auth_proto,dan1/django_openstack_auth_proto,redhat-openstack/django_openstack_auth,redhat-openstack/django_openstack_auth