priorityClass support for telegraf helm chart

Hi,

It would be good to add a `priorityClassName` field to the telegraf helm chart, similar to https://github.com/influxdata/helm-charts/pull/178.
This would help ensure the telegraf pod can be scheduled to k8s nodes with very high memory or cpu allocation.

Ref:
https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/#priorityclass

Compare the telegraf Helm chart with the referenced InfluxData pull request and read the linked Kubernetes PriorityClass documentation first. Add chart support for setting priorityClassName on the Telegraf pod, then verify that the rendered chart includes the requested value.

Written by the indexing model from the issue text.