message: [cluster-test] added instances_for_group api
  Closes:
  Approved by: andll
diff: #![forbid(unsafe_code)]
  -use crate::instance::Instance;
  +use crate::instance::{Instance, ValidatorGroup};
  use config_builder::ValidatorConfig;
  use libra_crypto::{
  ed25519::{Ed25519PrivateKey, Ed25519PublicKey},
  @@ -220,4 +220,12 @@ impl Cluster {
  self.validator_and_fullnode_instances()
  .find(|i| i.peer_name() == pod)
  }
  +
  + pub fn instances_for_group(
  + &self,
  + validator_group: ValidatorGroup,
  + ) -> impl Iterator<Item = &Instance> {
  + self.all_instances()
  + .filter(move |v| v.validator_group() == validator_group)
  + }
  }
  