fix: make zone in subnet optional

[ederst]

This makes setting zone in the ClusterSubnetSpec optional.

Technically, kOps does not require this to be set as it will also include the zones from the InstanceGroupSpec:

https://github.com/kubernetes/kops/blob/43cc54b2b1fcd768c7c4992989123541421e25a2/pkg/apis/kops/model/utils.go#L36-L50

fixes: #918