Reconfigure Vault provider for Terraform

Summary:
The provider actually reads the token from /.vault-token
when VAULT_TOKEN environment variable doesn't exist automatically.

In another hand, to avoid to have to set VAULT_ADDR can be useful.

This is a follow-up for 8e0a33bd332c.

Test Plan: terraform apply

Differential Revision: https://devcentral.nasqueron.org/D3921