De-duplicate Terraform configuration when possible

[invidian]

Currently each platform have a copy-pasted copy of e.g. Ignition configuration for controllers and workers, which makes it very difficult to keep a feature parity between all platforms, which include bug fixes as well.

This is a source of multiple problems and inconsistencies across platforms:

• Shutting down bare-metal nodes do not remove them from the cluster #448
• Unable to run OpenEBS on bare metal because iscsid.service is not enabled #447
• Replacing controller node on bare-metal platform does not re-join etcd member #446
• Don't copy kubeconfig over SSH for bare metal workers #444
• Reduce the amount of places where we define Kubernetes version #302

It should be possible to at least have a single Ignition template for both worker and controller, where we can add snippets with platform-specific or role-specific configuration. Such attempt has been done in 48d6f5e in #392.