EXOSCALE: finish implementation of zone creation

[das7pad]

Hi @pierre-emmanuelJ! While reviewing all the ZoneCreator implementations, I noticed that the EXOSCALE provider has an incomplete implementation for EnsureZoneExists. This PR is finishing the implementation, but I have no account with them for testing. Would you mind giving this a try and let me know how it goes? Thanks!

Part of https://github.com/StackExchange/dnscontrol/issues/3007