Refactor terraform-pipeline template

Example push pipeline - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/rk725-demo/infrastructure/-/pipelines/502717 Example merge request pipeline (although I had disabled the staging and production jobs in my test project but the development plan should be proof that it is working) - https://gitlab.developers.cam.ac.uk/uis/devops/experiments/rk725-demo/infrastructure/-/pipelines/502718

Closes #68 (closed)