We need to add `add_header` to proxy_vhost

I forgot that our vhost definitions in YAML didn't use the
nginx::resource::vhost resource and instead used our proxy_vhost. This
mean that deployment broke because we were not exposing the add_header
parameter. This commit fixes that by passing it through to the
underlying.
Merge pull request #544 from alphagov/fix-add-header

We need to add `add_header` to proxy_vhost