Add versioning for site provisioning

- Adds new input `provision_version` allowing us to gate the large
  refactor of terraform that is about to be underway
- Document the `provision_version` value in such a way that consumers of
  the **Orchestrator** project know it is not required unless they are
  contributing to the refactor of the **Provision** module

Signed-off-by: Robert Marshall <rmarshall@gitlab.com>