Upgrade Terraform to 0.12.29 & Add Gandi provider

Upgrade

• Upgrade default terraform version to 0.12.29

🆕 New

• New Gandi provider which exposes the two needed env variables (cf
  https://github.com/go-gandi/terraform-provider-gandi/blob/master/gandi/provider.go)

  • GANDI_SHARING_ID its a uuid found in the URL of your dashboard (when
    you are logged in to your organisation)
    e.g. if you dashboard is available at
    https://admin.gandi.net/dashboard/00000000-1111-2222-3333-444444444444/
    then your sharing id is 00000000-1111-2222-3333-444444444444
  • GANDI_KEY found in the “security” section of the account page
    https://account.gandi.net/

Fix

• Fix the Hetzner provider by using only a token for credentials (instead of key + secret)