kn-admin domain unset command

[ZhuangYuZY]

For issue https://github.com/knative/client-contrib/issues/29:

Now we have kn admin domain set to set custom-domain. But we are missing to unset the custom domain if it is not used or incorrect.

unset Knative route domain for service

For example:

To unset a route domain

kn admin domain unset --custom-domain mydomain.com

Usage: kn admin domain unset [flags]

Flags: -d, --custom-domain string Custom domain to unset -h, --help help for unset

Global Flags: --config string config file (default is $HOME/.config/kn/plugins/admin.yaml)

[knative-prow-robot]

Welcome @ZhuangYuZY! It looks like this is your first PR to knative/client-contrib 🎉

[knative-prow-robot]

Hi @ZhuangYuZY. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[zhanggbj]

/ok-to-test

[zhanggbj]

Hi @rhuss, I remember you have a contact to help us join Knative Org, would you please help to add @ZhuangYuZY in Knative Org, or any guide we can follow? Thanks!

[zhanggbj]

@ZhuangYuZY The PR looks good! And I think it's good to have the following updates with this PR.

1. The help massage and example in kn-admin [README] about unset.(https://github.com/knative/client-contrib/blob/master/plugins/admin/README.adoc)

[ZhuangYuZY]

@zhanggbj refining is done based on your comments. Thank you.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zhanggbj, ZhuangYuZY

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: - ~~[plugins/admin/OWNERS](https://github.com/knative/client-contrib/blob/master/plugins/admin/OWNERS)~~ [zhanggbj] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[zhanggbj]

/lgtm

[zhanggbj]

@ZhuangYuZY Great and Merged 👍