implement `restrict_kubeapi` flag

[mxmxchere]

In https://github.com/SovereignCloudStack/k8s-cluster-api-provider we had a flag restrict_kubeapi

https://docs.scs.community/docs/container/components/k8s-cluster-api-provider/doc/configuration#parameters-clusters this flag is currently missing here.

[eigenpaul]

If I'm not mistaken this is already implemented in all of the alpha and scs cluster-stacks, inside openstack-cluster-template.yaml. I'll test its functionality and go from there.

[eigenpaul]

Seems like we're missing it as a variable and patch inside cluster-class.yaml