Initial Kubevirt provider implementation

[xmudrii]

What this PR does / why we need it:

This PR implements the initial and very minimal Kubevirt provider for KubeOne. The purposes of this implementation is to mitigate the issue outlined in #3366, i.e. to allow using machine-controller and operating-system-manager for Kubevirt clusters.

I initially though about reading supported environment variables and putting them in the secret if cloud provider is none, but it has two very big problems:

• We would start deploying arbitrary environment variables to the cluster, that are very likely not even related to that cluster
• We don't have a way to validate what environment variables are needed, e.g. there's no way to know that we need KUBEVIRT_KUBECONFIG for the said cluster that uses cloud provider none
  • This can make a huge trouble if someone runs kubeone apply without setting the needed environment variables, in which case the value would be removed from the cluster and machine-controller would start failing

Because of the given reasons, I thought that it's much safer to implement a very minimalistic provider for Kubevirt that doesn't have Terraform integration and support for CCM/CSI, but that can be used as a way to deploy machine-controller and operating-system-manager in a safe and reliable way.

Which issue(s) this PR fixes:
xref #3366

What type of PR is this?
/kind feature

Special notes for your reviewer:

This is very initial Kubevirt provider implementation, we'll consider and discuss about extending it in future KubeOne releases.

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

Initial implementation of Kubevirt provider for KubeOne. At the moment, the provider only supports deploying machine-controller and operating-system-manager for Kubevirt-based clusters. Terraform integration, CCM, CSI, and other cloud provider specific components are not supported and/or deployed automatically.

Documentation:

TBD

/assign @kron4eg @moadqassem
/hold for discussion and lazy consensus

[kron4eg]

/approve
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: b8924464ebe0a3096cb869068669661f0f092826

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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