Get dns server address if host isn’t using systemd-resolved
    
    The worker containers aren’t using systemd-resolved so
    get_host_dns() can’t find the dns server causing tests that need
    it to fail. Add an alternative place to look for the dns server address
    that doesn’t rely on systemd-resolved.
    
    Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>