[Helm] ServiceAccount for secrets is defaulting to "default"

[echohack]

In general, we should either be requiring this field, or setting it to the account name that is set elsewhere in the values.yaml file. Otherwise, this can cause users who are deploying Pachyderm 2.0 to run into the following error on deployment.

failed to create secret: secrets is forbidden: User "system:serviceaccount:spacemaps-pachyderm-2:pachyderm" cannot create resource "secrets" in API group "" in the namespace "spacemaps-pachyderm-2"