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