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

.gitlab-ci/terraform.yml

@@ -95,6 +95,13 @@ tf-0.13.x-validate-aws:
     PROVIDER: aws
     CLUSTER: $CI_COMMIT_REF_NAME
 
+tf-0.13.x-validate-exoscale:
+  extends: .terraform_validate
+  variables:
+    TF_VERSION: 0.13.5
+    PROVIDER: exoscale
+    CLUSTER: $CI_COMMIT_REF_NAME
+
 tf-0.14.x-validate-openstack:
   extends: .terraform_validate
   variables:
@@ -116,6 +123,13 @@ tf-0.14.x-validate-aws:
     PROVIDER: aws
     CLUSTER: $CI_COMMIT_REF_NAME
 
+tf-0.14.x-validate-exoscale:
+  extends: .terraform_validate
+  variables:
+    TF_VERSION: 0.14.3
+    PROVIDER: exoscale
+    CLUSTER: $CI_COMMIT_REF_NAME
+
 # tf-packet-ubuntu16-default:
 #   extends: .terraform_apply
 #   variables: