Improve eventhub plugin documentation

- keep only EVENTHUB_CONNECTION_STRING in set influxdata#1 as name and namespace
  are taken from connection string
- add env vars set influxdata#3 to show how to connect to eventhub using service
  principal
- comment that env vars are ignored when connection_string option is
  specified
- comment on importance of EntityPath in connection_string
- use json as default data format, as it's used more often with
  eventhubs

plugins/inputs/eventhub_consumer/eventhub_consumer.go

@@ -69,8 +69,6 @@ func (*EventHub) SampleConfig() string {
   ## This requires one of the following sets of environment variables to be set:
   ##
   ## 1) Expected Environment Variables:
-  ##    - "EVENTHUB_NAMESPACE"
-  ##    - "EVENTHUB_NAME"
   ##    - "EVENTHUB_CONNECTION_STRING"
   ##
   ## 2) Expected Environment Variables:
@@ -79,8 +77,17 @@ func (*EventHub) SampleConfig() string {
   ##    - "EVENTHUB_KEY_NAME"
   ##    - "EVENTHUB_KEY_VALUE"
 
+  ## 3) Expected Environment Variables:
+  ##    - "EVENTHUB_NAMESPACE"
+  ##    - "EVENTHUB_NAME"
+  ##    - "AZURE_TENANT_ID"
+  ##    - "AZURE_CLIENT_ID"
+  ##    - "AZURE_CLIENT_SECRET"
+
   ## Uncommenting the option below will create an Event Hub client based solely on the connection string.
   ## This can either be the associated environment variable or hard coded directly.
+  ## If this option is uncommented, environment variables will be ignored.
+  ## Connection string should contain EventHubName (EntityPath)
   # connection_string = ""
 
   ## Set persistence directory to a valid folder to use a file persister instead of an in-memory persister
@@ -139,7 +146,7 @@ func (*EventHub) SampleConfig() string {
   ## 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"
+  data_format = "json"
   `
 }