Telegraf: Consider setting HOSTNAME using kubernetes downward api

[Altair-Bueno]

Expected behaviour

Telegraf uses the pod name as hostname by using the Kubernetes Downward API. This can be latter used as an unique and human understandable indentifier on multiple plugins, like client id's in Kafka producers and consumers.

Current behaviour

Hostname is manually set by using the env key on Telegraf values. By default, the env variable is set to "telegraf-polling-service", which is shared among all replicas.