OCPBUGS-30260: Set load balancer target nodes in request serving isolation mode

[csrwng]

What this PR does / why we need it:
Adds an annotation on the HostedCluster to allow setting target node labels for control plane router load balancers.
Modifies the request serving node scheduler to set this annotation when scheduling a HostedCluster on a set of nodes.
Only applies subnet annotation to private router loadbalancer, since public load balancers require public subnets.

Which issue(s) this PR fixes (optional, use fixes #<issue_number>(, fixes #<issue_number>, ...) format, where issue_number might be a GitHub issue, or a Jira story:
Fixes #OCPBUGS-30260

Checklist

• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.

[openshift-ci-robot]

@csrwng: This pull request references Jira Issue OCPBUGS-30260, which is valid.

3 validation(s) were run on this bug

• bug is open, matching expected state (open)
• bug target version (4.16.0) matches configured target version for branch (4.16.0)
• bug is in the state POST, which is one of the valid states (NEW, ASSIGNED, POST)

No GitHub users were found matching the public email listed for the QA contact in Jira (jiezhao@redhat.com), skipping review request.

The bug has been updated to refer to the pull request using the external bug tracker.

In response to this:

What this PR does / why we need it:
Adds an annotation on the HostedCluster to allow setting target node labels for control plane router load balancers.
Modifies the request serving node scheduler to set this annotation when scheduling a HostedCluster on a set of nodes.
Only applies subnet annotation to private router loadbalancer, since public load balancers require public subnets.

Which issue(s) this PR fixes (optional, use fixes #<issue_number>(, fixes #<issue_number>, ...) format, where issue_number might be a GitHub issue, or a Jira story:
Fixes #OCPBUGS-30260

Checklist

• Subject and description added to both, commit and PR.
• Relevant issues have been referenced.

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[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[csrwng]

/test verify
/test unit
/test e2e-aws

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: csrwng

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[openshift-ci]

@csrwng: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/e2e-aws	eb602df	link	true	/test e2e-aws

Full PR test history. Your PR dashboard.

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[stevekuznetsov]

Changes look reasonable, do we have any unit test coverage for these functions?

[openshift-merge-robot]

PR needs rebase.

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