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

[12345lcr]

For issue: #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.

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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.

[maximilien]

See comments. Thanks for contributions.

[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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[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

[navidshaikh]

How about just --secret instead of --secret-name ?

[zhanggbj]

@navid @12345lcr
I agree, it's good to keep the flag short. About the wording, we have an issue #53 to track, we can fix it there. Thanks!