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

[moadqassem]

/approve
/lgtm

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg, moadqassem

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [kron4eg,moadqassem]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[xmudrii]

/hold cancel