Set-up `gdsoperations/resolvconf` module

I'm using this module to set the contents of `/etc/resolv.conf` on a
machine-by-machine basis, scoped around the provider they're running on.

Install the module, and include it in Hiera (since we use this as a
lightweight ENC).
Set nameservers for provider0 (Skyscape)

Skyscape don't currently have nameservers that we can use in the IL0/2
environment, and we can't use Carrenza's nameservers because they only work
from within the Carrenza environment.

On that basis, and since there's nothing else we can use, set the
nameservers to use in Skyscape to Google Public DNS.
Set nameservers for provider1 (Carrenza)

Carrenza provide nameservers we can resolve against from within their
network. Set Carrenza-hosted machines to use them.
Use domain-specific Hiera backends
Merge pull request #41 from alphagov/provider-specific-nameservers

Set provider-specific nameservers