Fix failing unittests by including `BuildRequires: passwd`
    
    The unittests started failing because of upstream commit
    7c07af2 where cloud-init can now support using `usermod` to
    lock an account if `passwd` isn't installed. Since `passwd`
    wasn't installed in our mock buildroot it was choosing to
    use `usermod` and the unittests were failing.
    
    See: https://github.com/canonical/cloud-init/commit/7c07af2