Add retain flag to mqtt output to let brokers/servers cache measurements

[p-kraszewski]

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

---

Not unit-testable. Behavior confirmed to be correct in-house on the following configuration:

• FreeBSD 11-STABLE
• Mosquitto 1.5.3
• mosquitto.conf:

persistence true
autosave_interval 5
autosave_on_changes true
log_dest syslog
log_type warning
persistence_location /data/mosquitto/
persistent_client_expiration 1w

bind_address 127.0.0.1
port 1300

• telegraf.conf:

[global_tags]
[agent]
  interval = "60s"
  round_interval = true
  metric_batch_size = 100
  metric_buffer_limit = 10000
  collection_jitter = "500ms"
  flush_interval = "10s"
  flush_jitter = "10s"
  precision = ""
  quiet = true
  hostname = ""
  omit_hostname = false
[[outputs.mqtt]]
  servers = ["localhost:1300"]
  client_id = "telegraf-out"
  qos = 2
  batch = true
  retain = true
  topic_prefix = "telegraf"
[[inputs.cpu]]
[[inputs.disk]]
  ignore_fs = ["tmpfs", "devtmpfs", "devfs", "fdescfs", "procfs"]
[[inputs.diskio]]
  devices = ["ada*", "da*"]
[[inputs.kernel]]
[[inputs.mem]]
[[inputs.processes]]
[[inputs.swap]]
[[inputs.system]]
[[inputs.net]]
[[inputs.syslog]]
  server = "udp4://localhost:6514"
  keep_alive_period = "1m"
  best_effort = true

Broker caches results until they are drained by:

mosquitto_sub -p 1300 -v -t 'telegraf/#' -q 2

[danielnelson]

Follow up change for review comments: c08b454