Code review comment for ~smoser/ubuntu/+source/simplestreams:bug/xenial-1686437-keystone-v3

Also Noting here the released version on xenial does not currently support Keystone v3 and blocks Bug #1611987.

For the record, we have been running a bzr branch @455 on serverstack (a keystone v3 cloud) for months now. So the code in simplestreams works.