Allow configuring s3 etcd-snapshot-retention in config file

[maggie44]

Description

k3s has a number of options for configuring s3: https://docs.k3s.io/cli/etcd-snapshot#options

These can be configured by adding the flags, but ideally they would be added to the kube.tf config file as specific options.

This has become particularly pertinent as a PR introduced back in August (k3s-io/k3s#9866) changed the way s3 backups are done and for a standard 3 control plane cluster the default k3s value of 5 is likely insufficient for most as it no longer backs up all of the control planes for a sufficient time span. Instead, we may need to start scaling that number based on the number of control planes we run.

[mysticaltech]

@maggie44 PR please.

[maggie44]

Not forgotten, just not got to yet. 👍

[maggie44]

@mysticaltech, would:

 control_planes_custom_config = {
    "etcd-snapshot-retention" : 10,
  }

achieve this? The equivalent of passing in --etcd-snapshot-retention value? (https://docs.k3s.io/cli/etcd-snapshot#options)