fix(distros): clarify misleading ssh_redirect_user warning (#6906)

When ssh_redirect_user is set to true or default, cc_users_groups
resolves it to the actual default username before create_user runs.
The 'no cloud public-keys' warning then printed that resolved name
(e.g. 'ubuntu'), making it look like the user had configured 'ubuntu'.
Reword the warning so the value reads as the redirect target, not the
configured value.

Fixes GH-3924