Load balancer allowed CIDR is ignored during cluster provisioning

[andrewbonney]

Magnum UI surfaces an 'Allowed CIDR' label for the tenant cluster's k8s API access. magnum-cluster-api doesn't support this label, but given that it is silently ignored, this could give tenants a false sense of security with regard to their cluster's control plane, potentially leaving it open to all Internet traffic.

For the time being in our own deployment, we have added a check to validate_cluster in utils.py which prevents users launching clusters which make use of floating IPs against their control nodes.