@Override
  public String authenticate(InternalDistributedMember member, Properties credentials) {
    return authenticate(member, credentials, securityProps);
  }
Authenticate peer member with authenticator class defined by property
"security-peer-authenticator".

@param member the member to be authenticated
@param credentials the credentials used in authentication
@return null if authentication succeed (including no authenticator case), otherwise, return
        failure message
geode-core/src/main/java/org/apache/geode/distributed/internal/membership/adapter/auth/GMSAuthenticator.java