Fix reading configuration for multiple custom domains

Without this patch, the different domains end up with pointers to the same domainsConfig struct, as go re-uses the same region of memory on each iteration of a for loop.

Rework of !26 (closed) with added tests.

shared/pages/group/group.test.io/config.json

@@ -9,7 +9,7 @@
     {
       "Domain": "other.domain.com",
       "Certificate": "test",
-      "Certificate": "key"
+      "Key": "key"
     }
   ]
 }