Remove user-data plugins from machine-controller

[ahmedwaleedmalik]

User data plugins from machine-controller have been removed in favor of OSM.

What this PR does / why we need it:

Historically, machine-controller relied on user data plugins to generate configurations(cloud-config) for worker machines. Each operating system requires its own user-data plugin. These configs are then injected into the worker nodes using provisioning utilities such as cloud-init or ignition. Eventually, the nodes are bootstrapped.

Over time, it has been observed that this workflow has certain limitations.

• Machine Controller expects ALL the supported user-data plugins to exist and be ready. User might only be interested in a subset of the available operating systems. For example, user might only want to work with ubuntu.
• The user-data plugins have templates defined in-code. Which is not ideal since code changes are required to update those templates. Then those code changes need to become a part of the subsequent releases for machine-controller and KubeOne. So we need a complete release cycle to ship those changes to customers.
• Managing configs for multiple cloud providers, OS flavors and OS versions, adds a lot of complexity and redundancy in machine-controller.
• Since the templates are defined in-code, there is no way for an end user to customize them to suit their use-cases.
• Each cloud provider sets some sort of limits for the size of user-data, machine won't be created in case of non-compliance. For example, at the time of writing this, AWS has set a hard limit of 16KB.
• Better support for air-gapped environments is required.

To overcome these limitations; Kubermatic built Operating System Manager a few years ago. Please refer to Operating System Manager for more details.

These plugins were deprecated about a year ago in #1642.

User data plugins from machine-controller have been removed in favor of OSM.

Which issue(s) this PR fixes:

Fixes #1788

What type of PR is this?

/kind cleanup

Special notes for your reviewer:
There will be a follow-up PR with some more cleanup, moving things around from MC to OSM. But since it requires changes in OSM first, we'll do it later in another PR.

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

[ACTION REQUIRED] User data plugins for machine-controller have been removed. [Operating System Manager](https://github.com/kubermatic/operating-system-manager) is the successor of user data plugins. It's responsible for creating and managing the required configurations for worker nodes in a Kubernetes cluster with better modularity and extensibility. Please refer to [Operating System Manager](https://github.com/kubermatic/operating-system-manager#kubermatic-operating-system-manager) for more details.

Documentation:

https://docs.kubermatic.com/operatingsystemmanager

[ahmedwaleedmalik]

@xrstf pull-machine-controller-e2e-vsphere, pull-machine-controller-e2e-azure-custom-image-reference needs to be overridden.

Maybe in the future, we can run these tests periodically, once a day, so that we are more aware of issues like kubermatic/operating-system-manager#381 and generally failing tests like pull-machine-controller-e2e-azure-custom-image-reference

[kron4eg]

LOVE IT!

small change proposed

[kron4eg]

/approve
/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 081522b2ad4b314ff3be7fc7dfe94316ae16727a

[kubermatic-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: kron4eg

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• OWNERS [kron4eg]

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[kron4eg]

/lgtm cancel

there is failing linter job

[ahmedwaleedmalik]

/lgtm cancel

there is failing linter job

Should be fixed now, even more cleanup :D

[kron4eg]

/lgtm

[kubermatic-bot]

LGTM label has been added.

Git tree hash: 70398970b434ded79958a79173597d5a4f2d4de8

[xrstf]

/override pull-machine-controller-e2e-azure
/override pull-machine-controller-e2e-azure-custom-image-reference
/override pull-machine-controller-e2e-vsphere

[kubermatic-bot]

@xrstf: Overrode contexts on behalf of xrstf: pull-machine-controller-e2e-azure, pull-machine-controller-e2e-azure-custom-image-reference, pull-machine-controller-e2e-vsphere

In response to this:

/override pull-machine-controller-e2e-azure
/override pull-machine-controller-e2e-azure-custom-image-reference
/override pull-machine-controller-e2e-vsphere

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