kitchen: Use SealedSecret template for config

The configuration file for the kitchen HUD server has credentials
embedded in it.  Until I get around to refactoring it to read these from
separate locations, we'll make use of the template feature of
SealedSecrets.  With this feature, fields can refer to the (decrypted)
value of other fields using Go template syntax.  This makes it possible
to have most of the `config.yaml` document unencrypted and easily
modifiable, while still protecting the secrets.

kitchen/kitchen.yaml

@@ -54,8 +54,8 @@ spec:
           readOnly: true
       volumes:
       - name: config
-        configMap:
-          name: kitchen
+        secret:
+          secretName: kitchen
           optional: true
       - name: tzinfo
         hostPath: