Avoid empty "supersede domain-name-servers" directives for dhclient.conf

[spantaleev]

This patch aims to be minimal and intentionally:

• does not change the generation logic for supersede_domain and supersede_search
• does not change how nameserverentries (for NetworkManager) is built

It seems like nameserverentries in the "Generate nameservers for resolvconf, including cluster DNS" task is built the same way as dhclient_supersede_nameserver_entries_list. However, nameserverentries in the "Generate nameservers for resolvconf, not including cluster DNS" task (below) is built differently for some reason. It includes configured_nameservers as well. Due to these differences, I have refrained from reusing the same building logic (dhclient_supersede_nameserver_entries_list) for both.

If the configured_nameservers addition can be removed or made to apply to dhclient as well, we could potentially build a single list and then generate the nameserverentries and supersede_nameserver strings from it.

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixes a dhclient.conf corruption on etcd-only nodes on non-Network-Manager systems (Ubuntu 20.04, etc.)

Which issue(s) this PR fixes:

Fixes https://github.com/kubernetes-sigs/kubespray/issues/10947

Special notes for your reviewer:

None

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

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[yankay]

/ok-to-test

[spantaleev]

Review feedback has been addressed and the patch has been rebased (it was in a state of conflict after da3ff1cc117d688f55bd440fa8254d1bdc).

Perhaps it's ready for merging now?

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[ant31]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ant31, MrFreezeex, spantaleev

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