[kn-admin] Refine subcommand wording

[zhanggbj]

Per discussion in
#40 (comment)

now in kn-admin, for different subcommand domain/registry, we're using add/update/etc for create/update actions. We should be consistent in kn-admin.

kn client has a story to use create/apply/remove. We can follow the same way to keep same user experience.

[chaozbj]

/assign
I would like to work on it.

[zhanggbj]

@chaozbj
Could you please also take care of this #47 (comment)? Thanks!

[chaozbj]

@zhanggbj ok, no prblem

[chaozbj]

I would like to put it on hold because kn client has the similar feature and waits for implementation: knative/client#655, we will follow kn client to introduce create/apply/remove in our plugin.

[chaozbj]

The codes have been merged, will close it.
/close

[knative-prow-robot]

@chaozbj: You can't close an active issue/PR unless you authored it or you are a collaborator.

In response to this:

The codes have been merged, will close it.
/close

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