Pass Host header for custom errors

[lahsivjar]

What this PR does / why we need it: With custom-http-errors enabled the current nginx.tmpl will proxy_pass to http://upstream_balancer which will result in nginx rewriting the Host header with a value upstream_balancer in which _ is an invalid character as per [RFC952]: https://www.rfc-editor.org/rfc/rfc952.txt

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #3018

Special notes for your reviewer: Another way to fix it can be by not using an upstream name containing any invalid characters, however, I selected this approach since the Host header might be an useful information for the custom error backend.

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[lahsivjar]

/check-cla

[lahsivjar]

/assign @ElvinEfendi

[ElvinEfendi]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ElvinEfendi, lahsivjar

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[ElvinEfendi]

thanks @lahsivjar !