(backport) Feat: Add Terraform module

[mvlassis]

Ref: #270

This is a backport to create a Terraform module in the terraform directory. It follows the structure proposed in this spec.

To test the module:

• Clone the repository and switch to this branch.
• First run tox -e tflint to ensure that linting is correct
• Change to the terraform/ directory and run terraform init.
• Run terraform apply -var "channel=0.13/stable" -var "model_name=kubeflow" --auto-approve and wait until the charm is Active and Idle.

[mvlassis]

Closing since the backport is for branch track/0.13