protected String getProxyHost(String system) {
    return get(String.format(CONFIG_PROXY_HOST, system), DEFAULT_CONFIG_PROXY_HOST);
  }
Get the proxy host as a system level config. This is needed when
users need to go through a proxy for the Kinesis connections.
@param system name of the system
@return proxy host name or empty string if not defined
samza-aws/src/main/java/org/apache/samza/system/kinesis/KinesisConfig.java