tests

* kubeadm: do not ignore preflight errors blindly

The "ignoring all errors" seems to date back to the inception of the
kubeadm support (it was --skip-preflight-check before).

This can mask real errors and prevent users from seeing them.

Do not ignore any errors by default and make the set of ignored errors
configurable.

* download/kubeadm: remove redundant task

The mode is already set by the previous `copy` task.

* Validate kubeadm configs

This should help to fail early when we have invalid kubeadm configs (from
a kubespray bug or a misconfiguration).

* kubeadm-upgrade: remove unnecessary bool cast

* Convert kubeadm join discovery timeout to v1beta4 config

* CI: Ignore kubeadm:Mem errors on some setup.