network: allow LLDP packets to cross non-customer bridges for container network inter...

network: allow LLDP packets to cross non-customer bridges for container network interfaces

This changes the default .network files we ship for nspawn containers to set
EmitLLDP=customer-bridge in order to allow propagation of the LLDP packets
across bridges. This is useful so that "networkctl status" shows all peers
connected to a virtual container network, collecting this data via LLDP. This
is safe since the default configuration for these interfaces does not bridge
these links to external interfaces, but relies on IP routing for this.