Add conditional default subdomain setting to proxy

If a domain has been declared for the host, the upstream file will proxy
at a subdomain named for the stack it proxies to,
otherwise it will do the same for the path.

roles/whoami/templates/whoami_upstream.json.j2

@@ -1,13 +1,30 @@
 {
-  "@id": "{{ stack.name }}_upstream",
-  "handle": [
-    {
-      "handler": "reverse_proxy",
-      "upstreams": [
+    "@id": "{{ stack.name }}_upstream",
+{% if domain is not undefined and not none %}   
+    "match": [
         {
-          "dial": "{{ stack.name }}_app:80"
+            "host": [
+                "{{ stack.name }}.{{ domain }}"
+            ]
         }
-      ]
-    }
-  ]
+    ],
+{% else %}
+    "match": [
+        {
+            "path": [
+                "/{{ stack.name }}*"
+            ]
+        }
+    ],
+{% endif %}
+    "handle": [
+        {
+            "handler": "reverse_proxy",
+            "upstreams": [
+                {
+                    "dial": "{{ stack.name }}_app:80"
+                }
+            ]
+        }
+    ]
 }