kubefedcluster use cadata in kubeconfig file

[huiwq1990]

What this PR does / why we need it:

Kubefedcluster's cadata build from member cluster's serviceaccount, it work well when kubefed directlly conntect to member cluster. In my case, kubefed connect to a proxyserver (it proxy all apiserver's traffic), and the proxyserver has a independent cabundle.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

[RainbowMango]

It's an interesting use case.

@huiwq1990 I guess you joining cluster with a kubeconfig file which contains proxyserver's CA. The joining process is perfectly fine, but the following request to a member cluster will be disallowed as proxyserver failed to verify the certificate. Is that true?

[huiwq1990]

@RainbowMango Yes, kubefedcluster only use kubeconfig's apiserfver endpoint, it should use kubeconfig's cabundle too.

[irfanurrehman]

Looks legit to me. LGTM. @RainbowMango is this change allright with you?

[makkes]

This needs to be documented somewhere. I would suggest this page is the right place for it.

[huiwq1990]

@makkes @irfanurrehman I write a note specify the change.

[RainbowMango]

@irfanurrehman Yeah, I think the feature is reasonable. If there is CA data present in KUBECONFIG file, it is the preferred choice.
Thanks @huiwq1990
/retest

[hectorj2f]

/lgtm

[hectorj2f]

@makkes could you have a look again ? It looks good to me.

[hectorj2f]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hectorj2f, huiwq1990

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 [hectorj2f]

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