The new flag takes in a Terraform Enterprise API token and attempts to
write a config file for that token to ~/.terraformrc on startup.
If the file already exists and its contents would change as a result of
us writing it, we error out.
This flag is useful if you're using the TFE backend for any of your
projects as that requires a .terraformrc file with a token for
authentication.

In Terraform 0.12 and above, you aren't allowed to set -var foo=bar
flags unless the foo variable is actually defined in code. Previously,
we were setting the following variables:
- atlantis_user
- atlantis_repo
- atlantis_repo_owner
- atlantis_repo_name
- atlantis_pull_num

Users could then use these variables if they wanted to, in their code.
The main use case was to name the assume role session in AWS:

provider "aws" {
  assume_role {
    role_arn     = "arn:aws:iam::ACCOUNT_ID:role/ROLE_NAME"
    session_name = "${var.atlantis_user}-${var.atlantis_repo_owner}-${var.atlantis_repo_name}-${var.atlantis_pull_num}"
  }
}

This is longer possible in 0.12.