RBAC: fix chart to be consistent with others

This patch set does a minor correction so it is consistent with the
RBAC code in other charts.

Change-Id: I8c28af48a1d5a540fbc67b2dbcf4873081fc04bd
implements: bp/rbac-refactor

prometheus-alertmanager/templates/statefulset.yaml

@@ -43,7 +43,7 @@ spec:
         configmap-bin-hash: {{ tuple "configmap-bin.yaml" . | include "helm-toolkit.utils.hash" }}
         configmap-etc-hash: {{ tuple "configmap-etc.yaml" . | include "helm-toolkit.utils.hash" }}
     spec:
-      serviceAccount: alertmanager
+      serviceAccountName: {{ $serviceAccountName }}
       affinity:
 {{ tuple $envAll "alertmanager" "server" | include "helm-toolkit.snippets.kubernetes_pod_anti_affinity" | indent 8 }}
       nodeSelector: