Helm chart not adding service account name to celery beat pods

[paulinjo]

The serviceAccountName cannot be applied to the celery beat pods when deploying using the Helm chart.

How to reproduce the bug

1. Set the serviceAccountName and supersetCeleryBeat.enabled:true in the values.yml file for helm deployment
2. Deploy using helm

Expected results

serviceAccountName spec is applied to the celerybeat pod.

Actual results

serviceAccountName spec applied to all pods except for celerybeat.

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.