Yarn integration check fails incorrectly for HA mode setup

[xiaochuanyu]

Output of the info page

====================
Collector (v 5.18.1)
====================

  Status date: 2017-11-15 21:32:05 (0s ago)
  Pid: 3253
  Platform: Linux-4.9.51-10.52.amzn1.x86_64-x86_64-with-glibc2.3
  Python Version: 2.7.14, 64bit
  Logs: <stderr>, /var/log/datadog/collector.log, syslog:/dev/log

  Clocks
  ======

    NTP offset: -0.0001 s
    System UTC time: 2017-11-15 21:32:05.838106

  Paths
  =====

    conf.d: /etc/dd-agent/conf.d
    checks.d: /opt/datadog-agent/agent/checks.d

  Hostnames
  =========

 <REDACTED>

  Checks
  ======

    ntp (5.18.1)
    ------------
      - Collected 0 metrics, 0 events & 0 service checks

    disk (5.18.1)
    -------------
      - instance #0 [OK]
      - Collected 36 metrics, 0 events & 0 service checks

    network (5.18.1)
    ----------------
      - instance #0 [OK]
      - Collected 25 metrics, 0 events & 0 service checks

    yarn (5.18.1)
    -------------
      - instance #0 [ERROR]: 'Expecting value: line 1 column 1 (char 0)'
      - Collected 0 metrics, 0 events & 1 service check


  Emitters
  ========

    - http_emitter [OK]

====================
Dogstatsd (v 5.18.1)
====================

  Status date: 2017-11-15 21:32:06 (0s ago)
  Pid: 3250
  Platform: Linux-4.9.51-10.52.amzn1.x86_64-x86_64-with-glibc2.3
  Python Version: 2.7.14, 64bit
  Logs: <stderr>, /var/log/datadog/dogstatsd.log, syslog:/dev/log

  Flush count: 130488
  Packet Count: 1610314
  Packets per second: 1.9
  Metric count: 11
  Event count: 0
  Service check count: 0

====================
Forwarder (v 5.18.1)
====================

  Status date: 2017-11-15 21:32:05 (1s ago)
  Pid: 3249
  Platform: Linux-4.9.51-10.52.amzn1.x86_64-x86_64-with-glibc2.3
  Python Version: 2.7.14, 64bit
  Logs: <stderr>, /var/log/datadog/forwarder.log, syslog:/dev/log

  Queue Size: 1323 bytes
  Queue Length: 2
  Flush Count: 446822
  Transactions received: 335663
  Transactions flushed: 335661
  Transactions rejected: 0
  API Key Status: API Key is valid


======================
Trace Agent (v 5.18.1)
======================

  Pid: 3248
  Uptime: 1305665 seconds
  Mem alloc: 1057616 bytes

  Hostname:<REDACED>
  Receiver: localhost:8126
  API Endpoint: https://trace.agent.datadoghq.com

  Bytes sent (1 min): 0
  Traces sent (1 min): 0
  Stats sent (1 min): 0

Steps to reproduce the issue:

1. Setup a YARN cluster with Resource Manager in High Availability mode. In this setup, you have multiple Resource Manager nodes but only one is active and others are standby.
2. Run sudo /etc/init.d/datadog-agent info on any standby Resource Manager node.

Describe the results you received:

    yarn (5.18.1)
    -------------
      - instance #0 [ERROR]: 'Expecting value: line 1 column 1 (char 0)'
      - Collected 0 metrics, 0 events & 1 service check

Describe the results you expected: The check should not fail on a standby RM node. I was expecting this:

    yarn (5.18.1)
    -------------
      - instance #0 [OK]
      - Collected 0 metrics, 0 events & 1 service check