Re-enable proxy-append-forwarded-host for Backdrop

The backdrop admin app is broken - on sign-in it issues a 302 redirect to
http://admin.backdrop rather than
http://admin.performance.service.gov.uk

Suspect this is due to losing the ``proxy_append_forwarded_host`` config
parameter in the refactoring that occurred in this pull request:
https://github.com/alphagov/pp-puppet/pull/253
Merge pull request #262 from alphagov/reenable-proxy-append-forwarded-host

Re-enable proxy-append-forwarded-host for Backdrop