Unable to fetch image tags for Dockerhub private repo

[samb1729]

As of flux 1.2.0, the cache warmer is failing to fetch image tags from dockerhub private repos.

Example log output:

ts=2018-01-11T23:30:04.828130984Z caller=warming.go:154 component=warmer err="requesting tags: errors:\ndenied: requested access to the resource is denied\nunauthorized: authentication required\n"

The logs should also make it clear which image we're failing to fetch tags for.

[rade]

I think this has also broken the Weave Cloud Deploy tutorials

I am getting

ts=2018-01-12T11:22:36.887182409Z caller=warming.go:154 component=warmer err="requesting tags: Get https://<id>-simba01.environments.katacoda.com/v2/root/front-end/tags/list: denied: access forbidden"

[samb1729]

Fixed by #897