CNAME assets and spotlight subdomains to frontend

This gets around horrible routing issues in our environments.

I can't get pp-puppet working at the moment:

    * Base MAC address for eth0/NAT must be set. Contact box maintainer for more information.

But if you fancy eyeballing this or running it for me, I'd really appreciate it.

Expected result: two additional cnames in the `dnsmasq.d/10-internal-dns` file of the form:

    cname=www.preview.performance.service.gov.uk,frontend