Upgrade puppet-nginx version

In order to upgrade the version of nginx we need to include more than
just the nginx package. The upgraded version of puppet-nginx explicitly
sets the version of all packages nginx uses.
Move ppas out to a hiera variable

In order to experiment with a new nginx version in preview we need to be
able to define the ppas we install on to the system through heira.
Merge pull request #234 from alphagov/vary-nginx-version

Vary nginx version