[Reviewer: Matt] DNS fixes to various problems

This pull request:

* updates the bundle's DNS charm dependency to dns-2 (fixing a templating issue)
* sets ralf-hostname for ralf nodes, as the install fails otherwise
* appends to /etc/default/dnsmasq rather than overwriting it - overwriting it causes failures if the hook is invoked twice
* adds the Google Public DNS server 8.8.8.8 to dnsmasq.resolv.conf as well as the deployment's local BIND server. This means that "apt-get update" still works (where otherwise it fails because repo.cw-ngv.com etc. aren't resolvable).

I've tested these changes live (by checking that 'sprout.clearwater.local' is set up by the DNS charm, and that apt-get update works). I've also made a call through a quickstarted deployment.
Should we fix this up on all nodes, not just ralf?