WIP: fix: panic on gke private cluster creation

[salasberryfin]

/kind bug

What this PR does / why we need it:

#1416 introduced several changes to GCP API calls, as some fields have become deprecated. One of these changes causes a panic on a nil pointer as the controller is trying to assign a value to NetworkConfig.DefaultEnablePrivateNodes before NetworkConfig is even initialized. This was not an issue before as this private nodes setting was not part of NetworkConfig but PrivateClusterConfig.

NOTE

Together with this fix, I'll try to add tests for this reconciler so bugs like this one can be easily detected.

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 #1497

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Fix panic on GKE private cluster provisioning

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[cpanato]

thanks

[salasberryfin]

/hold

Still validating that this approach is correct

[mjura]

LGTM

[k8s-ci-robot]

@mjura: changing LGTM is restricted to collaborators

In response to this:

LGTM

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, mjura, salasberryfin

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[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[k8s-ci-robot]

PR needs rebase.

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