Fix pretty-printing of coredns and nodelocaldns configmaps

[VannTen]

What type of PR is this?
/kind bug

What this PR does / why we need it:
When using

dns_upstream_forward_extra_opts:
  prefer_udp: "" # the option as no value so use empty string to just
                 # put the key

This is rendered in the dns configmap as ($ for end-of-line)

...
prefer_udp $
...

Note the trailing space.
This triggers kubernetes/kubernetes#36222,
which makes the configmap hardly readable when editing them manually or
simply putting them in a yaml file for inspection.

Trim the concatenation of option + value to get rid of any trailing
space.
Example of kubectl edit on such a configmap:

  Corefile: ".:53 {\n    errors {\n    }\n    health {\n        lameduck 5s\n    }\n
    \   rewrite name suffix <redacted> cluster.local answer auto\n    ready\n    kubernetes
    cluster.local in-addr.arpa ip6.arpa {\n      pods insecure\n      fallthrough
    in-addr.arpa ip6.arpa\n    }\n    prometheus :9153\n    forward . <redacted>
    <redacted> {\n      prefer_udp\n      max_concurrent 1000\n      prefer_udp
    \n    }\n    cache 30\n\n    loop\n    reload\n    loadbalance\n}\n"

Special notes for your reviewer:
This is pretty minor, but pretty annoying !

Does this PR introduce a user-facing change?:

Fix pretty-printing (in kubectl) of nodelocaldns and coredns configmap when using `dns_upstream_forward_extra_opts` with an empty value option.

[VannTen]

/ok-to-test

[VannTen]

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

[k8s-infra-cherrypick-robot]

@VannTen: once the present PR merges, I will cherry-pick it on top of release-2.24, release-2.25, release-2.26 in new PRs and assign them to you.

In response to this:

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

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-sigs/prow repository.

[VannTen]

/cc @tico88612

[tico88612]

Thank you for fix the trailing spaces!

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tico88612, VannTen

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 [VannTen]

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

[k8s-infra-cherrypick-robot]

@VannTen: new pull request created: #11721

In response to this:

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

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-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@VannTen: new pull request created: #11722

In response to this:

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

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-sigs/prow repository.

[k8s-infra-cherrypick-robot]

@VannTen: new pull request created: #11723

In response to this:

/cherrypick release-2.26
/cherrypick release-2.25
/cherrypick release-2.24

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-sigs/prow repository.