message: [node] eliminate awkward counting of validator networks
  Closes:
  Approved by: davidiw
diff: @@ -221,7 +221,6 @@ pub fn setup_environment(node_config: &mut NodeConfig) -> LibraHandle {
  network_configs.push((RoleType::Validator, network_config));
  }
  - let mut validator_count = 0;
  // Instantiate every network and collect the requisite endpoints for state_sync, mempool, and consensus.
  for (role, network_config) in network_configs {
  // Perform common instantiation steps
  @@ -244,9 +243,12 @@ pub fn setup_environment(node_config: &mut NodeConfig) -> LibraHandle {
  match role {
  // Perform steps relevant specifically to Validator networks.
  RoleType::Validator => {
  - validator_count += 1;
  - // A valid config is allowed to have exactly one ValidatorNetwork
  - assert_eq!(1, validator_count);
  + // A valid config is allowed to have at most one ValidatorNetwork
  + // TODO: `expect_none` would be perfect here, once it is stable.
  + if consensus_network_handles.is_some() {
  + panic!("There can be at most one validator network!");
  + }
  +
  // Set up to listen for network configuration changes from StateSync.
  // TODO: move this inside network_builder.
  if let Some(conn_mgr_reqs_tx) = network_builder.conn_mgr_reqs_tx() {
  