`microcloud add` should exit if mandatory selections aren't made

When adding a new node and the existing cluster already has distributed networking (MicroOVN) and a `local` storage pool configured, it's mandatory that an uplink network's interface and storage disk is selected also for the new systems.

Right now this is optional and will cause errors later in the setup.

Instead MicroCloud should reject such selections and force the selection of the respective resources. We already know if we are running a bootstrap operation or adding new members and can use this as the base to identify how to proceed.