Argo workflow : set default service account

Hi !

For a reason I could not figure out, workflows without serviceaccountname does not work anymore on our `0.3.8` version (was working fine on `0.3.6`) as argoworkflows uses default serviceaccount which has no permissions.
We should set the default serviceaccount to `workflow` (which has enough permissions) by using workflowdefaults : https://github.com/argoproj/argo-helm/blob/c4d79885816f09214287e39cc36bee06d1411278/charts/argo-workflows/values.yaml#L194

```
workflowDefaults:
spec:
serviceAccountName: workflow
```

Review the referenced Argo Workflows values.yaml workflowDefaults section and locate the corresponding chart configuration entry point. Set the default service account to workflow, then verify that workflows without an explicit service account use it as expected for the 0.3.8 behavior described.

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