Fix path for kubelet.env file in kubelet deb package

[xmudrii]

What type of PR is this?

/kind bug

What this PR does / why we need it:

We received two reports that our kubelet Debian package installs kubelet.env file in a wrong location:

• Fix EnvironmentFile path for kubeadm deb package #3279 (comment)
• deb packages for kubelet on pkgs.k8s.io seem to include a systemd service definition for redhat machines #3276 (comment)

I was able to confirm these reports. As per some guidelines:

• /etc/default should be used on Debian-based operating systems
• /etc/sysconfig should be used on RPM-based operating systems

We partially fixed the issue a while ago with #3279, by ensuring that kubeadm is going to read the file from the correct path, but we didn't ensure that we install the file in the correct location. This is now fixed with this PR.

I tried building and installing the package locally to make sure the file is created in the correct location. This change will be applied to all kubelet packages that we build (for all minor releases) once this PR is merged. I don't expect this to regress in any way. 🤞

Does this PR introduce a user-facing change?

`kubelet.env` file is now installed in `/etc/default` instead of `/etc/sysconfig` on Debian-based systems by the `kubelet` package

/assign @saschagrunert @cpanato @puerco
cc @kubernetes/release-engineering

[xmudrii]

/priority important-soon

[xmudrii]

$ dpkg -L kubelet | grep /kubelet
/etc/default/kubelet
/lib/systemd/system/kubelet.service
/usr/bin/kubelet
/usr/share/doc/kubelet
/usr/share/doc/kubelet/LICENSE
/usr/share/doc/kubelet/README.md
/var/lib/kubelet
/var/lib/kubelet/.kubelet-keep

[saschagrunert]

Code LGTM, but the release note has a typo deafult

[xmudrii]

@saschagrunert Good point, fixed! 😅

[k8s-ci-robot]

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This pull-request has been approved by: cpanato, saschagrunert, xmudrii

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