Documentation should use endpoints with path /identity instead of port 5000

In devstack we configure keystone to run on port 80/443 proxied through the /identity URL path. We semi-officially recommend doing the same in production, but all of our documentation points to using port 5000 with no path. We should update the documentation to use the recommended endpoint configuration.

Note that keystone and horizon are commonly co-located and horizon by default runs on port 80/443 with no URL path, so the documentation will need to explain how to configure apache/nginx/haproxy such that horizon and keystone don't collide.