[kn-admin]Add support to remove registry with credentials

[12345lcr]

For issue: https://github.com/knative/client-contrib/issues/39

As a Knative administrator, I want to remove registry with credentials for Knative platform through username and server url.

For example:
# To remove a registry
kn admin registry remove --username=myusername  --server=docker.io

Usage:
  kn admin registry remove [flags]

Flags:
  -h, --help              help for remove
      --server string     Registry Address
      --username string   Registry Username

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

[knative-prow-robot]

Welcome @12345lcr! It looks like this is your first PR to knative/client-contrib 🎉

[knative-prow-robot]

Hi @12345lcr. 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.

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[zhanggbj]

/ok-to-test

[zhanggbj]

@12345lcr Generally it looks very good! Just a few minor comments as above. Also could you please help to update the examples for registry rmand user stories in https://github.com/knative/client-contrib/blob/master/plugins/admin/README.adoc? Thanks!

[zhanggbj]

@rhuss would you please help to add @12345lcr to knative org, or do you have the knative admin contract name? Thanks!

[rhuss]

You can ask in #steering-toc-questions or for now ping @evankanderson directly. I'm off because of holidays here in Germany and back on Monday.

[12345lcr]

@zhanggbj @chaozbj @maximilien Thank you all for reviewing and I am really appreciate it! I just updated the pr with comments addressed, feel free to review again and leave any comments.

[zhanggbj]

@12345lcr LGTM. But would you please also add ab example for registry rm in https://github.com/knative/client-contrib/blob/master/plugins/admin/README.adoc as others? Thanks!

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: 12345lcr, zhanggbj

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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 👍 just noticed @maximilien have requested for change, not sure If Max got further comments. If not, I'll merge it by end of today. Thanks @12345lcr for your PR and patience!!!

[zhanggbj]

/lgtm