Fix the QA K3s provider

What does this MR do?

We've introduced a ~bug to our K3s based QA tests in !64089 (comment 609846571).

This qa/qa/service/cluster_provider/k3s.rb is not a resource. So it inherits from QA::Service::ClusterProvider::Base, not QA::Resource::Base. This means there's no def init(&prepare_block) method available.

This MR basically reverts what got changed on this file.

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