use string for excludeFromBackup label

follow-up on !22 (merged) this MR makes sure the excludeFromBackup is a string. so volumes should be exluded when excludeFromBackup is set to "true" not to true

ping @jan-eike.schnabel

{{cookiecutter.project_slug}}/templates/{{cookiecutter.template_base_name}}/persistentvolumeclaim.yaml

@@ -11,7 +11,7 @@ metadata:
     io.kompose.service: {% raw %}{{ .Values.baseName }}{% endraw %}
     {%- raw %}
     {{- if .Values.volumeExcludeFromBackup }}
-    excludeFromBackup: true
+    excludeFromBackup: "true"
     {{- end }}
     {%- endraw %}
   name: {% raw %}{{ .Values.baseName }}{% endraw %}