Remove `node.serviceAccountName` property and fully replace with `serviceAccount.name`

[PierreBesson]

The service account name is already defined in serviceAccount.name, so it's strange that it's possible to redefine it in node.serviceAccountName and unclear what purpose it has.

Note apparently this is mandatory to set it to the right value for Vault auth to work:

        vault.hashicorp.com/role: {{ .Values.node.vault.authRole | default (include "node.serviceAccountName" .) | squote }}