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.

group_vars/testing.yml

@@ -5,4 +5,3 @@ docker_swarm_advertise_addr: eth1
 caddy_use_debug: yes
 caddy_use_https: no
 caddy_tls_use_staging: yes
-caddy_create_test_file_server: yes

roles/whoami/templates/whoami_upstream.json.j2

@@ -1,5 +1,22 @@
 {
     "@id": "{{ stack.name }}_upstream",
+{% if domain is not undefined and not none %}   
+    "match": [
+        {
+            "host": [
+                "{{ stack.name }}.{{ domain }}"
+            ]
+        }
+    ],
+{% else %}
+    "match": [
+        {
+            "path": [
+                "/{{ stack.name }}*"
+            ]
+        }
+    ],
+{% endif %}
     "handle": [
         {
             "handler": "reverse_proxy",