fix(misc): contrib/terraform/aws (#7818)

* fix(misc): terraform/aws

- handles deployment with a single availability zone
- handles deployment with more than two availability zone
- handles etcd collocation with control-plane nodes (`aws_etcd_num=0`)
- allows to set a bastion instances count (`aws_bastion_num`)
- allows to set bastion/etcd/control-plane/workers rootfs volume size
- removes variables from terraform.tfvars that were not re-used
- adds .terraform.lock.hcl to .gitignore
- changes/updates base image from ubuntu-18.03 to debian-10

tested by a few coworkers of mine, and myself: thanks for the outstanding
work, on both those terraform samples and kubespray playbooks.
I did not test ubuntu deployments, I could still swap from buster to
focal. LMK.

* fix(gitlab-ci)

AFAIU, terraform.tfvars indentation should be fixed for / no diff
returned running `terraform fmt -check -diff`

https://gitlab.com/kargo-ci/kubernetes-sigs-kubespray/-/jobs/1445622114