add vlan to 'clusterctl generate'

[lknite]

Describe the solution you'd like

vlan works when added to a network configuration in a cluster.yaml to be deployed via cluster-api, however, when creating a cluster using 'clusterctl generate' the 'VLAN' value does not make it into the cluster.yaml file.

Add 'VLAN' as an optional value to ~/.cluster-api/clusterctl.yaml & let it be used when running 'clusterctl generate'.

e.g.:

## -- networking configuration-- ##
CONTROL_PLANE_ENDPOINT_IP: "10.4.0.2"                         # The IP that kube-vip is going to use as a control plane endpoint
NODE_IP_RANGES: "[10.4.0.20-10.4.0.254]"                      # The IP ranges for Cluster nodes
GATEWAY: "10.4.0.1"                                           # The gateway for the machines network-config.
IP_PREFIX: "24"                                               # Subnet Mask in CIDR notation for your node IP ranges
DNS_SERVERS: "[10.0.0.2,10.0.0.3]"                            # The dns nameservers for the machines network-config.
BRIDGE: "vmbr0"                                               # The network bridge device for Proxmox VE VMs
VLAN: 1400                                                    # (Optional) VLAN Tag