Misleading ssh_redirect_user warning

[James Falcon]

On an Oracle instance containing this:
cat /etc/cloud/cloud.cfg.d/99-oracle-compute-user-redirect.cfg
# CLOUD_IMG: This file was created/modified by the Cloud Image build process
users:
  - default
  - name: opc
    ssh_redirect_user: true

We get a WARNING in the log with this:
__init__.py[WARNING]: Unable to disable SSH logins for opc given ssh_redirect_user: ubuntu. No cloud public-keys present.

Given that the user specified `ssh_redirect_user: true`, seeing `ubuntu` here is strange and misleading, even though the value has been changed for good reason.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3924