Log more details on cache warming errors

[marccarre]

Fixes #895.

With this PR, Flux now logs:

ts=2018-01-25T11:39:40.147644595Z \
  auth="{map[index.docker.io:<registry creds for marccarrecicd@index.docker.io, from default:secret/dockerhub-secret>]}" \
  caller=warming.go:154 \
  component=warmer \
  canonical_name=index.docker.io/marccarre/go-hello-world \
  err="requesting tags: Get https://index.docker.io/v2/marccarre/go-hello-world/tags/list: unauthorized: incorrect username or password"

Note that password is completely masked.

Not sure if we still want this PR though, given this comment and #897.

[squaremo]

#897 puts the logging behind a --registry-trace flag, and I think there's still value in also logging when there's been an error.

However, I'd request that it just omits passwords entirely -- I think it's more important to say where the credentials came from (struct cred now has a String method which summarises that) so people can investigate for themselves. Emitting any part of a password (as well as the length) is not sound.

[marccarre]

Thanks a lot for the feedback @squaremo, I'll rebase this branch on #897's one and make the suggested changes 👍

[squaremo]

Code review inline and request for change in the comment above (re omitting passwords).

[marccarre]

Also note that we now log additional details even when there is no error, e.g. when the registry is made public and the credentials are removed from the manifest:

ts=2018-01-25T11:58:41.349125409Z \
  caller=warming.go:190 \
  component=warmer \
  canonical_name=index.docker.io/marccarre/go-hello-world \
  creds={map[]} \
  fetching=marccarre/go-hello-world \
  total=2 \
  expired=2 \
  missing=0

ts=2018-01-25T11:58:41.481967446Z \
  caller=warming.go:238 \
  component=warmer \
  canonical_name=index.docker.io/marccarre/go-hello-world \
  creds={map[]} \
  updated=marccarre/go-hello-world \
  count=2

If too verbose, I can change the PR to only log canonical_name and creds on errors.
Let me know.

EDIT: Too verbose indeed.