Check on var controlplane_network_interface in BYOL

[akrzos]

It seems not actually in use, I am unsure of how we arrived at the conclusion it was to be added to the byol document.

[dbutenhof]

It seems that controlplane_network_interface is only referenced in the create-inventory template files. Since the BYOL playbooks assume a manually constructed inventory file and don't run create-inventory, those variables aren't used and shouldn't appear in the BYOL documentation.