feat(fluent-operator): added priorityClassName mapping to fluentd

[leonsteinhaeuser]

Unfortunately, the fluentd section does not currently support setting the priorityClassName field, which may be required in some environments. To solve this problem, I made this small change.

[benjaminhuo]

For PRs of the fluent operator chart, we recommend to submit to https://github.com/fluent/fluent-operator/tree/master/charts first, we'll sync each fluent operator release chart here in this repo