Make SSH keys DRY on all platforms

[ipochi]

This issue is about how to inherit the values of duplicate fields that exist in worker pools and controllers, if not provided by the user.

• Explicitly by the user, by either copy-pasting the same value in both the place or using a variable This would mean that worker_pool.ssh_pubkeys is required and must be provided by the user.

Example of an AWS configuration:

variable "ssh_public_keys {}"

cluster "aws" {
  ...
  ssh_pubkeys = var.ssh_public_keys
  ...

  worker_pool "p1" {
    ssh_pubkeys = var.ssh_public_keys
  }
}

• Implicitly by having lokoctl check if the field is provided or not and then inherit from the controller.

This would mean that worker_pool.ssh_pubkeys is optional and if not present in the configuration, the value is inherited from the controller configuration.

Current status in the supported non-managed platforms:

Packet: worker_pool.ssh_pubkeys field not present but there should be as part of feature and platform parity. AWS: worker_pool.ssh_pubkeys is required and explicitly duplicated by the user. Baremetal: no worker_pool concept yet.

[invidian]

I think it's fine to do the inheritance. I can imagine the case where different SSH keys are desired for worker nodes. It seems we can also use optional for []string in HCL, which allows passing empty array explicitly in the configuration, so SSH keys can be completely disabled for workers, if desired.

So I'd say:

1. require ssh_pubkeys to be not-empty: len(c.SSHPubKeys) > 0
2. Inherit controller SSH keys if worker keys are not defined in configuration: if workerPool.SSHPubKeys == nil { workerPool.SSHPubKeys = c.SSHPubKeys }.

[iaguis]

I agree with @invidian.

[johananl]

I agree with the above. To emphasize, I think the default behavior should be the simplest, most common use case (one key to rule them all :ring:) and at the same time the user should be able to specify key A for controllers, key B for worker pool 1 and key C for worker pool 2.

In terms of implementation, I'm for the "implicit approach" (worker pools use the controller keys unless specified otherwise).