Fix disabling NAT gateway creation for cluster role subnets

[cPu1]

What type of PR is this?
/kind bug

What this PR does / why we need it:

When creating an AzureCluster with a pre-existing single subnet shared by both control plane and worker nodes (role: cluster), the AzureCluster reconciler attempts to erroneously create a NAT gateway, eventually failing with a publicips failed to create or update. err: failed to get existing resource (service: publicips): parameter publicIPAddressName cannot be empty error.

This is a result of the defaulting webhook always setting the NAT gateway name even when a subnet ID is specified, resulting in an empty NATGatewayIP.Name.

The webhook updates this networkSpec

networkSpec:
  subnets:
  - name: cluster-subnet
    id: subnet-id
    cidrBlocks:
      - 10.0.1.0/24
    role: cluster

to

networkSpec:
  subnets:
  - name: cluster-subnet
    id: subnet-id
    cidrBlocks:
      - 10.0.1.0/24
    role: cluster
    natGateway:
      ip:
        name: ""
      name: cluster-nat-gateway

This changelist fixes the issue by setting a default NAT Gateway name only when subnet.ID is empty.

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 #5815

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• cherry-pick candidate

Release note:

Fixes disabling NAT gateway for `cluster` role subnets

[linux-foundation-easycla]

• ✅login: cPu1 / (dc78d0e)

The committers listed above are authorized under a signed CLA.

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[k8s-ci-robot]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.06%. Comparing base (72b27bb) to head (dc78d0e).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5816      +/-   ##
==========================================
+ Coverage   52.93%   53.06%   +0.13%     
==========================================
  Files         279      279              
  Lines       29666    29678      +12     
==========================================
+ Hits        15704    15750      +46     
+ Misses      13149    13112      -37     
- Partials      813      816       +3     

☔ View full report in Codecov by Sentry.
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[jackfrancis]

/ok-to-test

[jackfrancis]

/test ls

[k8s-ci-robot]

@jackfrancis: The specified target(s) for /test were not found.
The following commands are available to trigger required jobs:

/test pull-cluster-api-provider-azure-apiversion-upgrade

/test pull-cluster-api-provider-azure-build

/test pull-cluster-api-provider-azure-ci-entrypoint

/test pull-cluster-api-provider-azure-e2e

/test pull-cluster-api-provider-azure-e2e-aks

/test pull-cluster-api-provider-azure-test

/test pull-cluster-api-provider-azure-verify

The following commands are available to trigger optional jobs:

/test pull-cluster-api-provider-azure-apidiff

/test pull-cluster-api-provider-azure-apiserver-ilb

/test pull-cluster-api-provider-azure-capi-e2e

/test pull-cluster-api-provider-azure-conformance

/test pull-cluster-api-provider-azure-conformance-custom-builds

/test pull-cluster-api-provider-azure-conformance-dual-stack-with-ci-artifacts

/test pull-cluster-api-provider-azure-conformance-ipv6-with-ci-artifacts

/test pull-cluster-api-provider-azure-conformance-with-ci-artifacts

/test pull-cluster-api-provider-azure-conformance-with-ci-artifacts-dra

/test pull-cluster-api-provider-azure-dra-scalability

/test pull-cluster-api-provider-azure-e2e-optional

/test pull-cluster-api-provider-azure-e2e-workload-upgrade

/test pull-cluster-api-provider-azure-load-test-1k-dra-with-workload-custom-builds

/test pull-cluster-api-provider-azure-load-test-custom-builds

/test pull-cluster-api-provider-azure-load-test-dra-custom-builds

/test pull-cluster-api-provider-azure-load-test-dra-with-workload-custom-builds

/test pull-cluster-api-provider-azure-perf-test-apiserver-availability

/test pull-cluster-api-provider-azure-windows-custom-builds

/test pull-cluster-api-provider-azure-windows-with-ci-artifacts

Use /test all to run the following jobs that were automatically triggered:

pull-cluster-api-provider-azure-apidiff

pull-cluster-api-provider-azure-build

pull-cluster-api-provider-azure-ci-entrypoint

pull-cluster-api-provider-azure-e2e

pull-cluster-api-provider-azure-e2e-aks

pull-cluster-api-provider-azure-test

pull-cluster-api-provider-azure-verify

In response to this:

/test ls

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[jackfrancis]

/test pull-cluster-api-provider-azure-apiserver-ilb
/test pull-cluster-api-provider-azure-capi-e2e
/test pull-cluster-api-provider-azure-conformance-custom-builds
/test pull-cluster-api-provider-azure-load-test-dra-custom-builds
/test pull-cluster-api-provider-azure-apiversion-upgrade
/test pull-cluster-api-provider-azure-e2e-workload-upgrade

[jackfrancis]

/lgtm
/approve

/hold for tests

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 9d8d0f2186bee5179e651272893d1fb9ec2eb77f

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jackfrancis

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[k8s-ci-robot]

@cPu1: 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
pull-cluster-api-provider-azure-capi-e2e	dc78d0e	link	false	/test pull-cluster-api-provider-azure-capi-e2e

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[jackfrancis]

@mboersma here's another repro of failed capi-e2e, seems like this broke yesterday (though I don't see any obvious changes to CAPZ @ main that would be responsible)

[jackfrancis]

/hold cancel