rename httpd/keystone.py to httpd/keystone.wsgi

This matches openstack-dashboard[1]
.py is mislead since this isn't usable outside mod_wsgi

[1] https://github.com/openstack/horizon/tree/master/openstack_dashboard/wsgi

Change-Id: I1ba7f0e0f72d5c9fa1d06cec2b50551eb7f2e296
(cherry picked from commit 6ae00e0805b5cba9c92bf39472ce6615be127e8b)