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

See https://www.pivotaltracker.com/story/show/66497844 [#66497844]