Argo workflow : set default service account

[olevitt]

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