Add IgnorePreflightErrors to KubeadmConfig types

[sbueringer]

What steps did you take and what happened:

Based on Slack thread: https://kubernetes.slack.com/archives/C8TSNPY4T/p1619121490147500

We saw that there were some fields missing which are in upstream kubeadm v1beta2. At a first glance we're missing:

• CertificateKey
• IgnorePreflightErrors

The theory in Slack was that these fields were added after we forked the types

What did you expect to happen:
I would have expected to be able to confgure (at least) IgnorePreflightErrors.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Cluster-api version:
• Minikube/KIND version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

/kind bug
[One or more /area label. See https://github.com/kubernetes-sigs/cluster-api/labels?q=area for the list of labels]

[fabriziopandini]

Hopefully I can help with some context here:

1. Current version of Cluster API KubeadmConfig is derived from kubeadm API v1 beta1
2. Starting from v1alpha4, Cluster API KubeadmConfig types are decoupled from a specific version of the kubeadm API
3. Building on top of 2, it is now possible to implement some changes/cleanups in KubeadmConfig to improve usability

Accordingly

• IgnorePreflightErrors does not exists yet because this field was not present in v1beta1; I'm +1 to get this added, but we should be aware that this field will be no-op on older Kubernetes (this is not a problem IMO)
• CertificateKey was not present in v1beta1 too, however I don't think that we should add it to KubeadmConfig because we are not using automatic copy of certificates in Cluster API.

[sbueringer]

Makes sense to me. Is there already an issue open for v1alpha4 changes to the KubeadmConfig types or do we want to keep the current issue open to add the IgnorePreflightErrors field?

[fabriziopandini]

AFAIK there is no issue. It is ok to use this one for now.

[vincepri]

/retitle Add IgnorePreflightErrors to KubeadmConfig types
/assign @sbueringer
/milestone v0.4
/priority important-longterm

We should add validation that this field should only be used with Kubernetes v1.22+

[fabriziopandini]

NOTE: Validation can be tricky for Machine deployments because the Kubernetes version to use might be on a different resource (while on KCP instead everything is on the same resource)