write nova.conf [keystone_authtoken] section to avoid ERROR: Unauthorized (HTTP 401)

I was having errors like ERROR: Unauthorized (HTTP 401) in the latest devstack.  It turned out nova was sending HTTPS to keystone which was expecting HTTP.  The latest nova sets up [keystone_authtoken] section in nova.conf.sample, but lib/nova in devstack builds nova.conf from scratch.  So. I added a block to set up keystone_authtoken in lib/nova