Fix alertmanager serviceaccountname reference

Alertmanager's serviceaccountname was hardcoded instead of using
the common definition used in other charts. This simply brings the
chart in line with the others

Change-Id: I81fa6814217f2e422617379d5e3bf3629f660407

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: