[terraform] Update terraform.tfvars.example and use json format

Summary:
The terraform.tfvars file needed updating to reflect the latest variables and we now recommend users to use the json format.

Depends on D12875

Test Plan: terraform plan. The example is in the valid format

Reviewers: varun, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12877