terraform-openstack: Added possibility to enable dhcp flag critical on one interface

[Xartos]

What type of PR is this?

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/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

We wanted to be able to set the critical dhcp flag on the main interface to make the VM not releasing the IP on network hiccups. I also fixed a small "TODO" now when we've changed to the cloudinit_config resource instead of the template_file resource so we now default to an empty list instead of a empty string.

The change is opt-in and should not cause any changes for existing clusters.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

terraform-openstack: Added possibility to enable dhcp flag critical on one interface

[k8s-ci-robot]

Hi @Xartos. Thanks for your PR.

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[floryut]

@Xartos Thank you for the PR

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: floryut, Xartos

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[yankay]

@Xartos Thanks
/lgtm