Set a default value for dns_zone automatically

[ralgozino]

The dns_zone is used to determine the FQDN of the etcd nodes, and used as SAN in the certificates generated by kubeadm.

We don't have a default value and probably it can be inferred or detected better.

https://github.com/sighupio/fury-kubernetes-on-premises/blob/9a8ac0338f513dd171f5901d33bb928018d23b44/roles/etcd/templates/kubeadm-etcd.yml.j2#L7-L9

The input option could still be useful in some specific cases to override the default domain.