feat: Add terraform module

[orfeas-k]

Adds a terraform module, as designed in CC006 - Terraform modules organization in the Juju ecosystem spec. The module facilitates the deployment of argo-controller charm, using the Terraform juju provider.

Since the spec is not finalized yet, the PR may miss features/decisions that are still under discussion.

Parts where we deviate from the spec

• Omitted units from variables (inputs). This is because CKF charms have not been designed to function with more than one units.

Decisions that are still under discussion in the spec

• outputs.tf and variables.tf are (and should be) in alphabetical order as per Terraform style guide.
• outputs provides and requires endpoints.
• variable channel has null as default value to replicate native juju behaviour, which also asks Charmhub about the default channel, if it's not provided. The spec leaves that up to the developers.
• variable app_name has a default value, same as the one in juju_application.argo_controller.charm.name. This is because juju deploy native behavior uses the charm.name as the application name, if none is provided.

Additional implementation notes

• .gitignore: I added things that didn't need to be commited. Not 100% sure this covers everything.

CI

Regarding the CI, there is a comment in the spec by Ghislain where he mentioned that "the first implementation steps for this spec will be to create shared workflows". Thus, I think we shouldn't block too much on the CI right now. I 'd prefer if we removed it completely than spend too much time reviewing it, since it will change in the future. That being said, here are the implementation details about it.

• This is to be merged after feat: Setup TFLint for linting jobs charmed-kubeflow-workflows#58. I updated there the lint job in order to install tflint, which cannot be installed as a pip dependency.
• For running terraform fmt and terraform validate, we are using Telco team's reusable workflow . An example of its usage can be found here).
• CI is using reusable workflow to deploy terraform module from feat: Add terraform-deploy reusable workflow charmed-kubeflow-workflows#59

Closes #196

Referenced work

• Add terraform module mimir-worker-k8s-operator#74
• https://github.com/canonical/terraform-juju-sdcore/tree/main/modules/sdcore-control-plane-k8s

(optional) TO DO

• Create a reusable workflow under charmed-kubeflow-workflows similar to https://github.com/canonical/sunbeam-terraform/blob/main/.github/workflows/deploy.yml in order to make a simple deployment using the module

Testing

The terraform can be used with a terraform apply like the one below, using only the arguments that are needed.

terraform apply -var "app_name=argoo" -var "channel=3.4/stable" -var "model_name=kubeflow" -var "resources={oci-image: 3}"

[mvlassis]

Great work!

• Testing the linter with tox -ve tflint in the charms/argo-controller directory with no errors.
• Running terraform init, and then terraform apply -var "channel=latest/edge" -var "model_name=kubeflow" --auto-approve on the kubeflow model creates the argo-controller application from latest/edge and goes on an idle state as expected.