bugfix: prepare yurthub server tls config panic

[rambohe-ch]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage
/sig storage

/kind bug

What this PR does / why we need it:

• bug:
  if yurthub re-startup when network between cloud and edge disconnected, a panic will happen for preparing server tls config. and the panic like as following:

• reason:
  yurthub will use healthy server address and certificate under /var/lib/yurthub/pki dir to create rest.Config for https server. the code is here: https://github.com/openyurtio/openyurt/blob/master/pkg/yurthub/kubernetes/rest/config.go#L83-113
  and when cloud-edge network breaks down, healthy server will become true, so rest.Config will be nil, then panic happened.

• solution:
  we only need to prepare for rest.Config, and not need to connect remote server, so healthy server is not a requirement, so we add a needHealthyServer parameter for GetRestConfig() to create rest.Config whether healthy server should be set or not.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

[openyurt-bot]

@rambohe-ch: GitHub didn't allow me to assign the following users: your_reviewer.

Note that only openyurtio members, repo collaborators and people who have commented on this issue/PR can be assigned. Additionally, issues/PRs can only have 10 assignees at the same time.
For more information please see the contributor guide

In response to this:

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:
/kind bug
/kind documentation
/kind enhancement
/kind good-first-issue
/kind feature
/kind question
/kind design
/sig ai
/sig iot
/sig network
/sig storage
/sig storage

/kind bug

What this PR does / why we need it:

• bug:
  if yurthub re-startup when network between cloud and edge disconnected, a panic will happen for preparing server tls config. and the panic like as following:

• reason:
  yurthub will use healthy server address and certificate under /var/lib/yurthub/pki dir to create rest.Config for https server. the code is here: https://github.com/openyurtio/openyurt/blob/master/pkg/yurthub/kubernetes/rest/config.go#L83-113
  and when cloud-edge network breaks down, healthy server will become true, so rest.Config will be nil, then panic happened.

• solution:
  we only need to prepare for rest.Config, and not need to connect remote server, so healthy server is not a requirement, so we add a needHealthyServer parameter for GetRestConfig() to create rest.Config whether healthy server should be set or not.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rambohe-ch

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 [rambohe-ch]

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

[rambohe-ch]

/assign @Fei-Guo
/assign @zc2638

[Fei-Guo]

/approve
/lgtm

[zc2638]

/lgtm
I have passed the test in my environment.

[rambohe-ch]

/lgtm
I have passed the test in my environment.

@zc2638 Thanks for testing.