The apply-manifests role rendered the IPAddressPool from a template using
lb_pool_name, but applied the L2Advertisement from a hardcoded remote file
where the pool name was baked in as 'first-pool'. Setting lb_pool_name to
anything else left the L2Advertisement pointing at a non-existent pool, so
MetalLB assigned LoadBalancer IPs but never advertised them (services got an
external IP but were unreachable).
Template the L2Advertisement locally from lb_pool_name, mirroring the
IPAddressPool pattern, so both manifests always reference the same pool.
