contrib/terraform/exoscale: Rework SSH public keys (#7242)

* contrib/terraform/exoscale: Rework SSH public keys

Exoscale has a few limitations with `exoscale_ssh_keypair` resources.
Creating several clusters with these scripts may lead to an error like:

```
Error: API error ParamError 431 (InvalidParameterValueException 4350): The key pair "lj-sc-ssh-key" already has this fingerprint
```

This patch reworks handling of SSH public keys. Specifically, we rely on
the more cloud-agnostic way of configuring SSH public keys via
`cloud-init`.

* contrib/terraform/exoscale: terraform fmt

* contrib/terraform/exoscale: Add terraform validate

* contrib/terraform/exoscale: Inline public SSH keys

The Terraform scripts need to install some SSH key, so that Kubespray
(i.e., the "Ansible part") can take over. Initially, we pointed the
Terraform scripts to `~/.ssh/id_rsa.pub`. This proved to be suboptimal:
Operators sharing responbility for a cluster risk unnecessarily replacing resources.

Therefore, it has been determined that it's best to inline the public
SSH keys. The chosen variable `ssh_public_keys` provides some uniformity
with `contrib/azurerm`.

* Fix Terraform Exoscale test

* Fix Terraform 0.14 test

contrib/terraform/exoscale/variables.tf

@@ -1,13 +1,13 @@
-variable zone {
+variable "zone" {
   description = "The zone where to run the cluster"
 }
 
-variable prefix {
+variable "prefix" {
   description = "Prefix for resource names"
   default     = "default"
 }
 
-variable machines {
+variable "machines" {
   description = "Cluster machines"
   type = map(object({
     node_type = string
@@ -21,24 +21,24 @@ variable machines {
   }))
 }
 
-variable ssh_pub_key {
-  description = "Path to public SSH key file which is injected into the VMs."
-  type        = string
+variable "ssh_public_keys" {
+  description = "List of public SSH keys which are injected into the VMs."
+  type        = list(string)
 }
 
-variable ssh_whitelist {
+variable "ssh_whitelist" {
   description = "List of IP ranges (CIDR) to whitelist for ssh"
-  type = list(string)
+  type        = list(string)
 }
 
-variable api_server_whitelist {
+variable "api_server_whitelist" {
   description = "List of IP ranges (CIDR) to whitelist for kubernetes api server"
-  type = list(string)
+  type        = list(string)
 }
 
-variable nodeport_whitelist {
+variable "nodeport_whitelist" {
   description = "List of IP ranges (CIDR) to whitelist for kubernetes nodeports"
-  type = list(string)
+  type        = list(string)
 }
 
 variable "inventory_file" {