Refactor Terraform build_defs

:wave:, I've made this PR which rewrites most of the Terraform build_defs, that I wrote originally so that it is more widely supported.

Added support for var files (.tfvars).
Optional use of Terraform providers via the Terraform provider cache.
Support Please sandboxing and remote builds.
Support Terraform 0.15 and Terraform 1.0 (should also support future versions easily).
Simpler integration with other tools (e.g. terratest) via virtual environments.
Consolidate the Terraform tooling into a single script to simplify the installation.

thought-machine / pleasings