Improve PURGE request handling in Varnish

Prevent PURGE requests coming through Nginx. This depends on https://github.com/alphagov/puppet-nginx/pull/20

Also, we have logic that switches between the backdrop read and write APIs
based on the verb. This catches PURGE requests and allows them to be
passed on to get handled by the PURGE section below.

I have not added a corresponding pp-smokey regression test for this. I'm unsure of how to make a test that can be run locally as well as on the environment.