[BB-2006] Use a separate variable for the default fallback domain in Gandi account
Created by: lgp171188
This will be needed when the 'DEFAULT_INSTANCE_BASE_DOMAIN' is not set to the base domain present in the Gandi account.
Testing instructions:
- In the stage environment, set the
GANDI_DEFAULT_BASE_DOMAIN
environment variable and verify that the DNS records for an instance are created properly as subdomains of the stage subdomain and not directly as the subdomains of the production domain.