Add ed25519 SSH Key support

[jkroepke]

RSA keys are problematic in newer linux distros and should be avoided if possible.

Azure supports the usage of ED25519 keys which should be always prefer over RSA keys.

Ref: https://azure.microsoft.com/en-us/updates/v2/ssh-key-support-for-linux-vms

[rswrz]

Thanks for this improvement. Please refer to our Terraform Module Style Guide.

Also, please run terraform-docs . to update the README.md file.

[rswrz]

The azurerm provider at v3 does not support ssh-ed25519 SSH keys.

terraform {
  required_providers {
    azurerm = {
      source  = "hashicorp/azurerm"
      version = "~> 3.111"
    }
}

variables {
  authentication_type = "SSH"
}

$ terraform init -test-directory=tests/remote
Initializing the backend...
Initializing modules...
- test.tests.remote.main.setup_tests in tests/remote
Initializing provider plugins...
- Finding hashicorp/random versions matching ">= 3.0.0, ~> 3.0"...
- Finding hashicorp/tls versions matching ">= 4.0.0, ~> 4.0"...
- Finding azure/azapi versions matching ">= 1.14.0, ~> 1.14"...
- Finding hashicorp/azurerm versions matching ">= 3.111.0, ~> 3.111"...
- Installing hashicorp/random v3.6.3...
- Installed hashicorp/random v3.6.3 (signed by HashiCorp)
- Installing hashicorp/tls v4.0.6...
- Installed hashicorp/tls v4.0.6 (signed by HashiCorp)
- Installing azure/azapi v1.15.0...
- Installed azure/azapi v1.15.0 (signed by a HashiCorp partner, key ID 6F0B91BDE98478CF)
- Installing hashicorp/azurerm v3.116.0...
- Installed hashicorp/azurerm v3.116.0 (signed by HashiCorp)

$ terraform test -test-directory=tests/remote
tests/remote/main.tftest.hcl... in progress
  run "setup_tests"... pass
  run "deploy_module_linux_ssh"... fail
╷
│ Error: - the provided ssh-ed25519 SSH key is not supported. Only RSA SSH keys are supported by Azure
│
│   with azurerm_linux_virtual_machine.this[0],
│   on r-vm.tf line 7, in resource "azurerm_linux_virtual_machine" "this":
│    7: resource "azurerm_linux_virtual_machine" "this" {
│
╵
tests/remote/main.tftest.hcl... tearing down

The time has come (pretty fast ;)) that we need to raise the minimum version to v4.1.

terraform {
  required_providers {
    azurerm = {
      source  = "hashicorp/azurerm"
      version = "~> 4.1"
    }
}

variables {
  authentication_type = "SSH"
}

$ terraform init -test-directory=tests/remote
Initializing the backend...
Initializing modules...
- test.tests.remote.main.setup_tests in tests/remote
Initializing provider plugins...
- Finding hashicorp/random versions matching ">= 3.0.0, ~> 3.0"...
- Finding hashicorp/tls versions matching ">= 4.0.0, ~> 4.0"...
- Finding azure/azapi versions matching ">= 1.14.0, ~> 1.14"...
- Finding hashicorp/azurerm versions matching ">= 3.111.0, ~> 4.1"...
- Installing hashicorp/random v3.6.3...
- Installed hashicorp/random v3.6.3 (signed by HashiCorp)
- Installing hashicorp/tls v4.0.6...
- Installed hashicorp/tls v4.0.6 (signed by HashiCorp)
- Installing azure/azapi v1.15.0...
- Installed azure/azapi v1.15.0 (signed by a HashiCorp partner, key ID 6F0B91BDE98478CF)
- Installing hashicorp/azurerm v4.2.0...
- Installed hashicorp/azurerm v4.2.0 (signed by HashiCorp)

$ terraform test -test-directory=tests/remote
tests/remote/main.tftest.hcl... in progress
  run "setup_tests"... pass
  run "deploy_module_linux_ssh"... pass
tests/remote/main.tftest.hcl... tearing down
tests/remote/main.tftest.hcl... pass

Success! 2 passed, 0 failed.

We could keep the default key algorithm at RSA and stick with v3, but I think it is absolutely fine to go with v4 from here.

[rswrz]

The remote test is slow and somehow broken. I improved it in #20 and merged this changed here. Hopefully, now the test runs successfully.