Set appropriate host header for proxying API request
Set appropriate host header for proxying API request

These were going from frontend nginx to backend nginx but backend nginx didn't have the data it
needed to pick the right vhost to serve the request. Setting the Host header seems to resolve
that.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet