providers: add Gandi as provider with their ENV Variables

The gandi providers expects two 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 dashbaord (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/

terraform.sh

@@ -158,6 +158,11 @@ while IFS=',' read -ra providers; do
                     declare -x "OVH_CONSUMER_KEY=${!token}"
                 fi
                 ;;
+            gandi)
+                if [ -z "${GANDI_SHARING_ID}" ]; then
+                    declare -x "GANDI_SHARING_ID=${!key}"
+                    declare -x "GANDI_KEY=${!secret}"
+                fi
         esac
 
     done