Fix machine selector config to allow kubelet arg list

[a-blender]

Issue: #1074

Problem

Machine Selector Config kubelet-arg values could not be passed via Terraform to downstream machines and would not show up in the rancher UI.

Solution

My solution is to convert machine_selector_config.config from Type.Map (string map) to Type.String that supports yaml like Machine Global Config + state migration logic to handle the schema update. This allows for users to input both strings and lists, which allows subfield kubelet-arg to be passed as a list which is what Rancher is expecting. It also prevents a potential regression caused by 1) only allowing string args or lists which will reduce the flexibility of Machine Selector Config or 2) defining kubelet-arg as a subfield which will break other config fields that are already expected in the Rancher backend such as private-default-registry.

Before, passing kubelet-arg under Machine Selector Config would not work. This feature now works with the following configuration

machine_selector_config {
  config = <<EOF
    protect-kernel-default: true
    kubelet-arg:
      - key1=value1
      - key2=value2
  EOF
}

Testing

Engineering Testing

Manual Testing

Test plan

• Prov rke2 cluster with Machine Selector Config and 2 kubelet args set. Verify cluster provisions successfully and args are displayed correctly in the rancher ui
• Add/remove a kubelet-arg via tf and verify list is updated in the ui
• Prov rke2 cluster with tf 3.1.0. Add machine selector config with 2 kubelet args via the rancher ui
• Upgrade tf to rc version with fix
• Verify no errors appear on terraform plan/apply or updates to Machine Selector Config.

Automated Testing

Tests updated. Ran go test -v ./rancher2 to make sure all automated tests pass.

QA Testing Considerations

Regressions Considerations

[jakefhyde]

Consider this a lgtm once @felipe-colussi's comments are addressed.

Lgtm once other comments were addressed