Enable image pull secret for BESS UPF chart

Change-Id: I48eecb5d29462b286d78086ad677574a39065670
diff --git a/omec/omec-user-plane/templates/statefulset-upf.yaml b/omec/omec-user-plane/templates/statefulset-upf.yaml
index 3ac3239..0bf4cfa 100644
--- a/omec/omec-user-plane/templates/statefulset-upf.yaml
+++ b/omec/omec-user-plane/templates/statefulset-upf.yaml
@@ -49,9 +49,9 @@
       nodeSelector:
         {{ .Values.nodeSelectors.upf.label }}: {{ .Values.nodeSelectors.upf.value }}
     {{- end }}
-    {{- if .Values.images.credentials }}
+    {{- if hasKey .Values.images "pullSecrets" }}
       imagePullSecrets:
-        - name: {{ .Release.Name }}.registry
+{{ toYaml .Values.images.pullSecrets | indent 8 }}
     {{- end }}
       initContainers:
       - name: bess-init