[release/v1.8] Add an option to disable Flatcar auto-upgrades

[xmudrii]

What this PR does / why we need it:

Manual cherry-pick of #3391 and #3398

Which issue(s) this PR fixes:
xref #2320

What type of PR is this?
/kind flake

Special notes for your reviewer:

The purpose of this PR is to ensure we can use this improvement in the upgrade tests on the main branch.

Does this PR introduce a user-facing change? Then add your Release Note here:

Add `disable_auto_update` option to example Terraform configs for AWS, Azure, Equinix Metal, OpenStack, and vSphere, used to disable automatic updates for all Flatcar nodes

Documentation:

NONE

[xmudrii]

/assign @kron4eg

[xmudrii]

/kind flake

[xmudrii]

/retest

[kron4eg]

/approve

[kubermatic-bot]

LGTM label has been added.

Git tree hash: beff9206a210da18b72a3509e3cf00170d97a0e3

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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[xmudrii]

/remove-label flake
/kind feature

[kubermatic-bot]

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/remove-label flake
/kind feature

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