Add NATS output plugin.

Added NATS server container needed for tests.

Added NATS output plug-in. Fixes #1487

NATS output plug-in use internal.GetTLSConfig to instrument TLS configuration.

Added NATS output plug-in to changelog.

closes #1487
closes #1697

plugins/outputs/nats/README.md

@@ -0,0 +1,37 @@
+# NATS Output Plugin
+
+This plugin writes to a (list of) specified NATS instance(s).
+ 
+```
+[[outputs.nats]]
+  ## URLs of NATS servers
+  servers = ["nats://localhost:4222"]
+  ## Optional credentials
+  # username = ""
+  # password = ""
+  ## NATS subject for producer messages
+  subject = "telegraf"
+  ## Optional TLS Config
+  ## CA certificate used to self-sign NATS server(s) TLS certificate(s)
+  # tls_ca = "/etc/telegraf/ca.pem"
+  ## Use TLS but skip chain & host verification
+  # insecure_skip_verify = false
+
+  ## Data format to output.
+  ## Each data format has it's own unique set of configuration options, read
+  ## more about them here:
+  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_OUTPUT.md
+  data_format = "influx"
+```
+
+### Required parameters:
+
+* `servers`:  List of strings, this is for NATS clustering support. Each URL should start with `nats://`.
+* `subject`: The NATS subject to publish to.
+
+### Optional parameters:
+
+* `username`: Username for NATS
+* `password`: Password for NATS
+* `tls_ca`: TLS CA
+* `insecure_skip_verify`: Use SSL but skip chain & host verification (default: false)