Add Kafka 0.9+ consumer support (#2487)

plugins/inputs/kafka_consumer_legacy/README.md

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+# Kafka Consumer Input Plugin
+
+The [Kafka](http://kafka.apache.org/) consumer plugin polls a specified Kafka
+topic and adds messages to InfluxDB. The plugin assumes messages follow the
+line protocol. [Consumer Group](http://godoc.org/github.com/wvanbergen/kafka/consumergroup)
+is used to talk to the Kafka cluster so multiple instances of telegraf can read
+from the same topic in parallel.
+
+## Configuration
+
+```toml
+# Read metrics from Kafka topic(s)
+[[inputs.kafka_consumer]]
+  ## topic(s) to consume
+  topics = ["telegraf"]
+  ## an array of Zookeeper connection strings
+  zookeeper_peers = ["localhost:2181"]
+  ## Zookeeper Chroot
+  zookeeper_chroot = ""
+  ## the name of the consumer group
+  consumer_group = "telegraf_metrics_consumers"
+  ## Offset (must be either "oldest" or "newest")
+  offset = "oldest"
+
+  ## Data format to consume.
+  ## Each data format has its own unique set of configuration options, read
+  ## more about them here:
+  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
+  data_format = "influx"
+
+  ## Maximum length of a message to consume, in bytes (default 0/unlimited);
+  ## larger messages are dropped
+  max_message_len = 65536
+```
+
+## Testing
+
+Running integration tests requires running Zookeeper & Kafka. See Makefile
+for kafka container command.