Model name validation error doesn't specify model name

As seen in http://reports.vapour.ws/releases/issue/5757063f749a561bd992d765.

Our CI tests used to contain underscores which caused this error to appear. However, despite the fact our model name is akin to maas-1_9-deploy-trusty-amd64, the error message displayed is:

"name" is not a valid name: model names may only contain lowercase letters, digits and hyphens

Obviously "name" is perfectly valid according to what's listed in the message, so this should display the actual name that is an issue.

-----------
if !names.IsValidModelName(cfg.mustString(NameKey)) {
  return fmt.Errorf("%q is not a valid name: model names may only contain lowercase letters, digits and hyphens", NameKey)
 }