If server_tokens is disabled completely remove the Server header

[bfin]

What this PR does / why we need it:

Actually removes the Server header when server-tokens: false, instead of just setting a null value.

This PR makes use of the more_clear_headers directive instead of the more_set_headers directive, both of which are included in the headers-more-nginx-module that the controller is already built with. The more_set_headers directive is currently used to set a null value for the Server header, but the more_clear_headers directive can completely remove it.

Module reference: https://github.com/openresty/headers-more-nginx-module#more_clear_headers

Which issue this PR fixes

Supersedes PR #1903 to fix #1094 more thoroughly.

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: [email protected]

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[bfin]

/check-cla

[codecov-io]

Codecov Report

Merging #2696 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2696   +/-   ##
=======================================
  Coverage   40.93%   40.93%           
=======================================
  Files          72       72           
  Lines        5084     5084           
=======================================
  Hits         2081     2081           
  Misses       2715     2715           
  Partials      288      288

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4f44ddc...d59e3ee. Read the comment docs.

[aledbf]

@bfin this LGTM. Once you fixed the CLA issue I'll merge this

[bfin]

/check-cla

[aledbf]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, bfin

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aledbf]

@bfin thanks!

[bfin]

@aledbf Small PR, but wow that was quick!