File output writing to standard streams are closed after reload

[danielnelson]

Bug report

Relevant telegraf.conf:

[[outputs.file]]
  files = ["stdout"]
  data_format = "influx"

System info:

Telegraf vdev-85-g7a5d8578 (git: master 7a5d857

Steps to reproduce:

1. Start telegraf
2. pkill -HUP telegraf

Expected behavior:

Telegraf reload configuration and continues sending metrics

Actual behavior:

Telegraf cannot write to output because it is closed.

Additional info:

2017-03-10T22:38:30Z E! Error writing to output [file]: failed to write message: cpu,host=loaner,cpu=cpu0 usage_iowait=0,usage_irq=0,usage_guest=0,usage_guest_nice=0,usage_user=4.124748490945898,usage_idle=95.1710261569454,usage_softirq=0,usage_steal=0,usage_system=0.7042253521128432,usage_nice=0 1489185040000000000
, write /dev/stdout: file already closed